Add keybinds to control global notifications

Alt+M: toggle "mute all notifications except highlights"
Alt+Shift+M: toggle "mute all notifications"

The Keys.Rooms.(previous/next)_highlight keybinds, previously bound to
Alt+(Shift+)M (stood for "mention") now defaults to Alt+(Shift+)H.

The NotificationLevel enum and notificationLevel property had to be
moved from Window to UI due to QML having a global "Window" object that
causes conflicts when trying to access the enum as
"Window.NotificationLevel" from UI.qml.

docs/TODO.md

@@ -6,7 +6,6 @@
 - explain pattern
 - fix spinbox buttons
 - HMenuItem checkbox styling
-- config & keybind for global rule disabling
 - quick settings
 - import/export/json edit rules?
 - fix flickable popups can't be flicked by keyboard