Make TasksApplication.gui expect an IGUI interface, not a GUI instance (

#301)

* TST : Test that a TraitError is raised

because the TasksApplication expects it's gui trait to be an instance
of GUI object and not an object that supports the IGUI interface

	modified:   envisage/ui/tasks/tests/test_tasks_application.py

* FIX : Make TasksApplication.gui use Supports IGUI

Not Instance GUI

	modified:   envisage/ui/tasks/tasks_application.py
	modified:   envisage/ui/tasks/tests/test_tasks_application.py

* FIX : Readd removed unittest skip decorator

and remove an unused import

	modified:   envisage/ui/tasks/tests/test_tasks_application.py

* CLN : Update test method name

	modified:   envisage/ui/tasks/tests/test_tasks_application.py

* REF : Use Instance, not Supports for gui trait

envisage/ui/tasks/tasks_application.py

@@ -60,7 +60,7 @@ class TasksApplication(Application):
     active_window = Instance("envisage.ui.tasks.task_window.TaskWindow")
 
     # The Pyface GUI for the application.
-    gui = Instance("pyface.gui.GUI")
+    gui = Instance("pyface.i_gui.IGUI")
 
     # Icon for the whole application. Will be used to override all taskWindows
     # icons to have the same.

envisage/ui/tasks/tests/test_tasks_application.py

@@ -18,13 +18,20 @@
 
 from envisage.ui.tasks.api import TasksApplication
 from envisage.ui.tasks.tasks_application import DEFAULT_STATE_FILENAME
+from pyface.i_gui import IGUI
+from traits.api import HasTraits, provides
 
 requires_gui = unittest.skipIf(
     os.environ.get("ETS_TOOLKIT", "none") in {"null", "none"},
     "Test requires a non-null GUI backend",
 )
 
 
+@provides(IGUI)
+class DummyGUI(HasTraits):
+    pass
+
+
 @requires_gui
 class TestTasksApplication(unittest.TestCase):
     def setUp(self):
@@ -119,3 +126,9 @@ def test_layout_load_pickle_protocol_3(self):
 
         state = app._state
         self.assertEqual(state.previous_window_layouts[0].size, (492, 743))
+
+    def test_gui_trait_expects_IGUI_interface(self):
+        # Trivial test where we simply set the trait
+        # and the test passes because no errors are raised.
+        app = TasksApplication()
+        app.gui = DummyGUI()