Bind keyPressEvent to dialog in configuration

When attempting to assign keyboard shortcuts for TTS playback, Mac OS X
is unable to capture the key press event if bound to the buttons. (The
buttons are unfocused after being clicked on that platform, so the key
press event might not be able to reach the buttons.)

Instead, we can try to bind the key press event to the dialog box
itself, which seems to always be able to catch the event. In this
variation, we can also check to see if both buttons are awaiting the
event, and assign both if so (now that using the same keyboard shortcut
on both the fronts and backs of cards is supported).
Allow use of escape key again on preferences

This needs to be handled within the key press handler, since it's not
in-place for the entire window.
Dead code removal from main module

These bits are generally alternate ways to do things that didn't work
out when they were originally tried (e.g. use of glob functionality).
Credit for Dusan Arsenijevic in main module

This is for the bit of Google "take a break" code added back in 2012,
which was originally missed since it's somewhat buried and not directly
reflected in the Git history.
Merge branch 'bug/mac-config-keys' into develop

Brings in fixes so that the shortcut keys can be properly remapped on
Mac OS X.