This prevents a startup failure caused by trying to set
the UI scale when .meta() is not working. Startup still
fails, but the correct message is displayed now.
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)
