Create menubar prototype

Some stuff still needs to be fixed:

- The close window option always closes the main window, even if it is not the currently active window (only applicable on systems with native menu bar)
- None of the (text) editing actions are enabled
- Actions related to instances should only be active when an instance is selected
- The open wiki option ("PolyMC Help") needs to be implemented
- Delete instance keyboard shortcut does not seem to work on my system. Test further
- It would be nice if the profiles menu had all of the logged in accounts, and if they could be selected from that menu (preferably with keyboard shortcuts, probably Ctrl + 1, Ctrl + 2, ...)

launcher/ui/MainWindow.h

@@ -110,6 +110,8 @@ private slots:
 
     void on_actionReportBug_triggered();
 
+    void on_actionOpenWiki_triggered();
+
     void on_actionMoreNews_triggered();
 
     void newsButtonClicked();