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.
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