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.
Add (buggy) support for delaying [sound] tags

This is the initial pass on this, with a few buggy bits:

- keystrokes still trigger the delay
- using the add/edit note window during a review session means anything
  that uses the native sound function (which, as far as I know, is just
  the playback of a newly-inserted sound), will be incorrectly delayed
- filenames that end up with a junky " (1)" filename won't match the
  regex and will not be recognized as AwesomeTTS sounds
Detect replay shortcut and add media in review

During reviews, the delays on media files will no longer be done if the
user is in the middle of inserting audio or strikes the replay shortcut.
Correctly handle incremented filenames

Sometimes an AwesomeTTS filename if it already exists can end up with an
index after its name; the regex for these should still recognize such as
a valid AwesomeTTS filename.
Rename/rewrite documentation for Playback config

Replaces the on-the-fly tab configuration page with a playback one,
with new information about the delay functionality and differences
between stored and on-the-fly playback.
Swap in new configuration images

These replacements include the newly-done Playback tab.
Add support for page redirects

This will initially be used to bump users hitting /config/on-the-fly
over to /config/playback.
Shortcut configuration fix for Mac OS X

With the introduction of text-like input fields on the first tab of the
configuration window, Mac OS X's implementation of Qt now cannot capture
keystrokes from the shortcut buttons.

This fix makes it so that a focus event is automatically fired on the
event buttons to focus them explicitly so that the keyboard events can
be captured.
Merge branch 'add/playback-delays-35' into develop

Adds support for the user to delay the playback of on-the-fly playback,
AwesomeTTS sounds, and non-AwesomeTTS sounds. These options are
configurable by sound type and by the side of the card (yielding 6
options total). These are available via the first tab of the
configuration screen, now renamed "Playback" (instead of "On-the-Fly").

Closes #35.