Added doxyfile (doxygen) for the common folder. Started with doxygen comments for app and tft module.

common/app/screen_main.h

@@ -1,3 +1,22 @@
 #include "screen.h"
 
+/**
+ * @addtogroup screens
+ */
+/*@{*/
+
+/**
+ * @defgroup main Main
+ * The Main Screen is the start-screen for the application
+ */
+/*@{*/
+
+
+/**
+ * Returns a pointer to the main screen
+ * \sa gui_screen_navigate
+ * @return
+ */
 SCREEN_STRUCT* get_screen_main();
+
+/*@}@}*/