Introduce configuration keys for delay support

This includes six keys total: for each card side, we are to support
delaying on-the-fly playback, stored audio generated from AwesomeTTS,
and stored audio from some other source.
Make room on configuration screen for delay

Renames the existing on-the-fly tab, removes the additional copy now
covered on the website, and expands the existing labels on that tab to
make it clear that they are on-the-fly/tts tag options.
Add new options to configuration

This makes quite a few adjustments layout-wise to the first tab to make
it all work and make some sense.
On-the-fly delay support

Adds support for the delay_questions_onthefly and delay_answers_onthefly
configuration options by putting a new lambda method in-between the
GUI implementations and the actual Anki sound interface.