Abstract out media file storage to Anki

This removes repeated code for unicode conversions and unlinking, and
will also enable special logic in the future for conditional unlinking.
Improve speed of Google record() when cached

During the record() hook, first check to see if we already have the
image locally in the cache directory. If so, pass that path to Anki for
inclusion in the media collection.

Additionally, makes adjustments to various places in the main module to
prevent removal of the cached asset and do not apply throttling when
retrieving cached assets.
Adjustments to AwesomeTTS button hook

Improvements to tooltip and removal of setCheckable(), the latter of
which was causing the button to remain depressed after being clicked
initially.