Make offline presence to stop sync

Setting the presence of an account to offline
will make the client to end sync task and will
prevent messages from being sent.

Setting it to online again or any other presence
will start sync task again.

Left:
- Local echo to presence change
- UI Control to affect all members presence
- Block more requests to be sent (e.g. member
  actions)

src/gui/Pages/Chat/RoomPane/RoomPane.qml

@@ -57,6 +57,10 @@ MultiviewPane {
     }
 
 
+    readonly property QtObject accountModel:
+        ModelStore.get("accounts").find(chat.roomInfo.for_account)
+
+
     function toggleFocus() {
         if (roomPane.activeFocus) {
             if (roomPane.collapse) roomPane.close()
@@ -78,7 +82,9 @@ MultiviewPane {
     }
 
     MemberView {}
-    SettingsView {}
+    SettingsView {
+        enabled: accountModel.presence !== "offline"
+    }
 
     HShortcut {
         sequences: window.settings.keys.toggleFocusRoomPane