Some work towards supporting find/replace in our QScintilla-based widget (#261).
Some minor cleaning up.
Some minor cleaning up [ci skip].
Some minor improvements to our GUI interface.

This arose from trying to debug something by creating a test view that
doesn’t inherit from CommonWidget, which someone else might very well
decide to do too, so…
Some minor cleaning up [ci skip].
Some work towards supporting find/replace in our QScintilla-based widget (#261).
Some work towards supporting find/replace in our QScintilla-based widget (#261).
Some work towards supporting find/replace in our QScintilla-based widget (#261).
Some work towards supporting find/replace in our QScintilla-based widget (#261).
Some minor cleaning up [ci skip].
Prevent our tab bars (in our central widget) from getting the focus.

Indeed, in some cases, when pressing the Tab key a tab bar may get the
focus while we really don’t want that (it doesn’t look ‘neat’,
especially on OS X).
Some work towards supporting find/replace in our QScintilla-based widget (#261) [ci skip].
Some minor cleaning up.
Some work towards supporting find/replace in our QScintilla-based widget (#261).
Slight update to our developer documentation [ci skip].
Some work towards supporting find/replace in our QScintilla-based widget (#261).
Some minor cleaning up [ci skip].
Some work towards supporting find/replace in our QScintilla-based widget (#261).
Some minor cleaning up [ci skip].
Fixed the issue with the wrong message being displayed after adding a term directly and removing it straightaway (closes #310).