qtui: Rework the attention state behavior of the tray icon

qtui: Rework the attention state behavior of the tray icon

Move the attention behavior handling into the SystemTray base class,
which now determines the correct icon names to display based on
the tray state and notification settings. Systray implementations
now just react on signals from the base class to update their
icons accordingly.

Extend the systray notification settings to allow for a more fine-
grained configuration. Users can now choose if the tray should be
alerted at all, and if so, if the tray icon should change color or
even blink instead of relying on the visualizer's default.

This now works with both StatusNotifierItem and the legacy tray.

Supersedes and closes GH-327.