Common Filter class

Provides a new class in the GUI package for use with Qt's
installEventFilter functionality.
Patch reviewer to work with an event filter

This enables us to not use a private method and also removes the
conflict with other add-ons that do choose to use a wrapper around that
method.
Merge branch 'bug/otf-shortcut-conflict-25'

Brings in fix for how the on-the-fly shortcuts are bound to the
application, this version preferring a filter on the main window.

Fixes #25.