Add global notification control icon in top bar

The "Mentions & keywords only" option doesn't work yet in multi-account
cases: when notifications arrive for a room that multiple accounts are
part of, the first notification to arrive is let through, and the
duplicate later ones are ignored. The first notification to arrive may
not be from the account that gets highlighted, thus it is ignored under
the "Mentions & keywords" setting.

src/gui/MainPane/TopBar.qml

@@ -2,6 +2,7 @@
 
 import QtQuick 2.12
 import QtQuick.Layouts 1.12
+import ".."
 import "../Base"
 
 Rectangle {
@@ -69,12 +70,60 @@ Rectangle {
 
         HButton {
             backgroundColor: "transparent"
-            icon.name: "reload-config-files"
-            toolTip.text: qsTr("Reload config files")
 
-            onClicked: mainUI.reloadSettings()
+            icon.name:
+                window.notificationLevel === Window.NotificationLevel.All ?
+                "notifications-all" :
+
+                window.notificationLevel === Window.NotificationLevel.None ?
+                "notifications-none" :
+
+                "notifications-mentions-keywords"
+
+            icon.color:
+                window.notificationLevel === Window.NotificationLevel.All ?
+                theme.icons.colorize :
+
+                window.notificationLevel === Window.NotificationLevel.None ?
+                theme.colors.negativeBackground :
+
+                theme.colors.middleBackground
+
+            toolTip.text: qsTr("Control global notifications")
+            onClicked: notificationsMenu.open()
 
             Layout.fillHeight: true
+
+            HMenu {
+                id: notificationsMenu
+                y: parent.height
+
+                HMenuItem {
+                    icon.name: "notifications-all"
+                    text: qsTr("Normal notifications")
+                    onTriggered:
+                        window.notificationLevel =
+                            Window.NotificationLevel.All
+                }
+
+                HMenuItem {
+                    icon.name: "notifications-mentions-keywords"
+                    icon.color: theme.colors.middleBackground
+                    text: qsTr("Mentions & keywords")
+                    onTriggered:
+                        window.notificationLevel =
+                            Window.NotificationLevel.MentionsKeywords
+                }
+
+                HMenuItem {
+                    icon.name: "notifications-none"
+                    icon.color: theme.colors.negativeBackground
+                    text: qsTr("Nothing")
+                    onTriggered:
+                        window.notificationLevel =
+                            Window.NotificationLevel.None
+                }
+            }
         }
 
         HButton {