Update icon name

adwaita-icon-theme has dropped many old icon symlinks.
Always update the existing notification

The notifications show what's playing right now. It simply does not make
any sense to keep displaying old ones, especially if it actually delays
the current song information.

Only two places used the "issue new notification" notify(), and they
were inconsistent with similar code anyway.
Set notification hints just once when initializing
Remove too-obvious docs
Update metadata for same song, if called by idle event

Ref #60
Only send notifications if updated metadata is different