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

common/app/screen_guitest.h

@@ -1,3 +1,23 @@
 #include "screen.h"
 
+
+/**
+ * @addtogroup screens
+ */
+/*@{*/
+
+/**
+ * @defgroup guitest Guitest
+ * The Gui-Test Screen tests the gui and the tft module.
+ */
+/*@{*/
+
+
+/**
+ * Returns a pointer to the guitest screen
+ * \sa gui_screen_navigate
+ * @return
+ */
 SCREEN_STRUCT* get_screen_guitest();
+
+/*@}@}*/