If a user enables an old add-on and restarts immediately, it won't have
had a chance to update, and may raise errors that could have been
prevented if the user was running the latest version.
- web requests done on a background thread
- easier to use outside of the addon dialog
- gets max point version info from AnkiWeb, which we can use in
the future
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)
