Setting it straight away causes the cursor to flash on quick operations,
like saving the current note. Delay it for 300ms, which should hopefully
be long enough to not get in the way, but short enough to give indication
that long-running requests are being processed.
For things like updating the deck list, it makes no sense to do it
multiple times at the end of a long operation, and the once/second signal
timer was causing hundreds or thousands of 100ms timers to be created.
https://forums.ankiweb.net/t/anki-memory-usage/3842
Also:
- provide a way for the progress handler to skip the throttling so that
we can ensure progress is updated at the end of a stage
- show 'checking' at the end of full sync
This code was an awful hack to provide some semblance of UI
responsiveness while executing DB statements on the main thread.
Instead, we can just run DB statements in a background thread now,
keeping the UI responsive.
We were previously relying on the DB progress hook to cause the
progress window to display.
Qt's progress dialogs do have built in support for automatically
showing, but it's easier to add a timer than change the existing
code to use it.
Instead, set the busy cursor immediately to give the user feedback,
but defer popup for at least 500ms. This will hopefully address the
white flash in night mode on Windows, and prevent progress dialogs
from rapidly appearing and disappearing for short operations.
