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.

src/gui/UI.qml

@@ -6,12 +6,22 @@ import QtQuick.Controls 2.12
 import QtQuick.Layouts 1.12
 import QtQuick.Window 2.12
 import QtGraphicalEffects 1.12
+import "."
 import "Base"
 import "MainPane"
 
 Item {
     id: mainUI
 
+    enum NotificationLevel { Mute, HighlightsOnly, Enable }
+
+    property int notificationLevel:
+        settings.Notifications.start_level === "highlights_only" ?
+        UI.NotificationLevel.HighlightsOnly :
+        settings.Notifications.start_level === "mute" ?
+        UI.NotificationLevel.Mute :
+        UI.NotificationLevel.Enable
+
     property bool accountsPresent:
         ModelStore.get("accounts").count > 0 || py.startupAnyAccountsSaved
 
@@ -78,6 +88,25 @@ Item {
         }
     }
 
+    HShortcut {
+        sequences: window.settings.Keys.notifications_highlights_only
+        onActivated:
+            mainUI.notificationLevel =
+                mainUI.notificationLevel ===
+                UI.NotificationLevel.HighlightsOnly ?
+                UI.NotificationLevel.Enable :
+                UI.NotificationLevel.HighlightsOnly
+    }
+
+    HShortcut {
+        sequences: window.settings.Keys.notifications_mute
+        onActivated:
+            mainUI.notificationLevel =
+                mainUI.notificationLevel === UI.NotificationLevel.Mute ?
+                UI.NotificationLevel.Enable :
+                UI.NotificationLevel.Mute
+    }
+
     FontMetrics {
         id: fontMetrics
         font.family: theme.fontFamily.sans