Various small CoregistrationUI improvements extracted from #10242 (#1…

…0245)

* Use overwrite='read' when checking for file to read

* Picking fiducials -> Placing MRI fiducials

* Limit dock width to 350px

* Set file filter and initial directory for trans file pickers

mne/gui/_coreg.py

@@ -2,6 +2,7 @@
 from functools import partial
 import os
 import os.path as op
+from pathlib import Path
 import time
 import queue
 import threading
@@ -284,7 +285,7 @@ def _get_default(var, val):
 
     def _set_subjects_dir(self, subjects_dir):
         self._subjects_dir = _check_fname(
-            subjects_dir, overwrite=True, must_exist=True, need_dir=True)
+            subjects_dir, overwrite='read', must_exist=True, need_dir=True)
 
     def _set_subject(self, subject):
         self._subject = subject
@@ -296,7 +297,7 @@ def _set_fiducials_file(self, fname):
         if not self._check_fif('fiducials', fname):
             return
         self._fiducials_file = _check_fname(
-            fname, overwrite=True, must_exist=True, need_dir=False)
+            fname, overwrite='read', must_exist=True, need_dir=False)
 
     def _set_current_fiducial(self, fid):
         self._current_fiducial = fid.lower()
@@ -314,15 +315,15 @@ def _set_info_file(self, fname):
             self._widgets["info_file"].set_value(0, '')
             return
 
-        fname = _check_fname(fname, overwrite=True)  # convert to str
+        fname = _check_fname(fname, overwrite='read')  # convert to str
 
         # ctf ds `files` are actually directories
         if fname.endswith(('.ds',)):
             self._info_file = _check_fname(
-                fname, overwrite=True, must_exist=True, need_dir=True)
+                fname, overwrite='read', must_exist=True, need_dir=True)
         else:
             self._info_file = _check_fname(
-                fname, overwrite=True, must_exist=True, need_dir=False)
+                fname, overwrite='read', must_exist=True, need_dir=False)
 
     def _set_omit_hsp_distance(self, distance):
         self._omit_hsp_distance = distance
@@ -466,7 +467,7 @@ def _lock_fids_changed(self, change=None):
         else:
             self._forward_widget_command(locked_widgets, "set_enabled", False)
             self._forward_widget_command(fits_widgets, "set_enabled", False)
-            self._display_message("Picking fiducials - "
+            self._display_message("Placing MRI fiducials - "
                                   f"{self._current_fiducial.upper()}")
         self._set_sensors_visibility(self._lock_fids)
         self._forward_widget_command("lock_fids", "set_value", self._lock_fids)
@@ -486,7 +487,7 @@ def _current_fiducial_changed(self, change=None):
         self._update_fiducials()
         self._follow_fiducial_view()
         if not self._lock_fids:
-            self._display_message("Picking fiducials - "
+            self._display_message("Placing MRI fiducials - "
                                   f"{self._current_fiducial.upper()}")
 
     @observe("_info_file")
@@ -1101,7 +1102,9 @@ def _check_fif(self, filetype, fname):
         return True
 
     def _configure_dock(self):
-        self._renderer._dock_initialize(name="Input", area="left")
+        self._renderer._dock_initialize(
+            name="Input", area="left", max_width="350px"
+        )
         mri_subject_layout = self._renderer._dock_add_group_box("MRI Subject")
         self._widgets["subjects_dir"] = self._renderer._dock_add_file_button(
             name="subjects_dir",
@@ -1237,7 +1240,9 @@ def _configure_dock(self):
         )
         self._renderer._dock_add_stretch()
 
-        self._renderer._dock_initialize(name="Parameters", area="right")
+        self._renderer._dock_initialize(
+            name="Parameters", area="right", max_width="350px"
+        )
         mri_scaling_layout = \
             self._renderer._dock_add_group_box(name="MRI Scaling")
         self._widgets["scaling_mode"] = self._renderer._dock_add_combo_box(
@@ -1353,6 +1358,8 @@ def _configure_dock(self):
             input_text_widget=False,
             tooltip="Save the transform file to disk",
             layout=save_trans_layout,
+            filter='Head->MRI transformation (*-trans.fif *_trans.fif)',
+            initial_directory=str(Path(self._info_file).parent),
         )
         self._widgets["load_trans"] = self._renderer._dock_add_file_button(
             name="load_trans",
@@ -1361,6 +1368,8 @@ def _configure_dock(self):
             input_text_widget=False,
             tooltip="Load the transform file from disk",
             layout=save_trans_layout,
+            filter='Head->MRI transformation (*-trans.fif *_trans.fif)',
+            initial_directory=str(Path(self._info_file).parent),
         )
         self._widgets["reset_trans"] = self._renderer._dock_add_button(
             name="Reset",