Added an option to save a project as the default template

A new option to save a project as the default template is now available
in the file menu. If the default template already exists the user is
asked whether he wants to overwrite it.

include/MainWindow.h

@@ -113,6 +113,7 @@ public slots:
 	bool saveProject();
 	bool saveProjectAs();
 	bool saveProjectAsNewVersion();
+	void saveProjectAsDefaultTemplate();
 	void showSettingsDialog();
 	void aboutLMMS();
 	void help();