Add support for a second keybind per action

Modifying code:
 - terminatorlib/config.py
 - terminatorlib/keybindings.py
 - terminatorlib/plugin.py
 - terminatorlib/preferences.glade
 - terminatorlib/prefseditor.py
 - terminatorlib/terminal_popup_menu.py
 - terminatorlib/window.py

Modifying test:
 - tests/test_prefseditor_keybindings.py

Updating documentation in doc/* to reflect dual keybinding options

Updating translation files in po/*

Closes #371

doc/terminator.1

@@ -1,7 +1,7 @@
 '\" t
 .\"     Title: terminator
 .\"    Author: [see the "AUTHOR(S)" section]
-.\" Generator: Asciidoctor 2.0.18
+.\" Generator: Asciidoctor 2.0.20
 .\"      Date: 2023-04-01
 .\"    Manual: Manual for Terminator
 .\"    Source: Terminator
@@ -181,7 +181,8 @@ replace hide keyboad shortcut.
 .SH "KEYBINDINGS"
 .sp
 The following default keybindings can be used to control Terminator.
-Most of these keybindings can be changed in the Preferences.
+Most of these keybindings can be changed in the Preferences. Up to
+two keybindings can be set per action.
 .sp
 \fBF1\fP
 .RS 4
@@ -482,4 +483,4 @@ Terminator was written by Chris Jones <cmsj@tenshu.net> and others.
 This manual page was written by Chris Jones <cmsj@tenshu.net> and others.
 .SH "SEE ALSO"
 .sp
-\fBterminator_config\fP(5)
+\fBterminator_config\fP(5)

doc/terminator.adoc

@@ -37,7 +37,7 @@ Start with a maximised window.
 Start with a fullscreen window.
 
 *-b*, *--borderless*::
-Instruct the window manager not to render borders/decoration on the
+Instruct the window manager not to render borders/decorations on the
 Terminator window (this works well with --maximise).
 
 *-H*, *--hidden*::
@@ -115,7 +115,8 @@ replace hide keyboad shortcut.
 
 == KEYBINDINGS
 The following default keybindings can be used to control Terminator.
-Most of these keybindings can be changed in the Preferences.
+Most of these keybindings can be changed in the Preferences. Up to
+two keybindings can be set per action.
 
 *F1*::
 Launches the full HTML manual.