Other areas of Anki don't confirm cancelling actions with tooltips,
so after further consideration, the tooltip felt out of the place and
might have actually confused users into thinking some action was
being performed.
Allows Anki to register a mime-type handler for .ankiaddon files
Other small collateral changes:
+ fix positioning issues with some prompts and progress dialog
+ add prompt titles where they were missing
+ add type annotations for AddonManager installation methods
+ explicitly import os in main (used to be imported via aqt.qt)
