qt_themes: add themed connected/disconnected icons for dark and fix status bar theme updating

In dark theme, it is very hard to see the connected/disconnected icons because they are dark too. So I added two white-coloured icons for the dark theme. This also fixed an issue where theme update does not change the icon on the status bar.

src/citra_qt/multiplayer/state.h

@@ -46,6 +46,7 @@ public slots:
     void OnOpenNetworkRoom();
     void OnDirectConnectToRoom();
     void OnAnnounceFailed(const Common::WebResult&);
+    void UpdateThemedIcons();
 
 signals:
     void NetworkStateChanged(const Network::RoomMember::State&);