GUI: abolish API injection into instances. (#265)

* GUI: abolish API injection into instances. Update usage by 3rd party apps.
* GUI: update documentation. Cleanup api usage. Adjust status bar item spacing.

applications/gui/icon_i.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "icon.h"
+
 #include <stdint.h>
 
 struct IconData {
@@ -16,6 +18,12 @@ struct Icon {
     uint32_t tick;
 };
 
+/*
+ * Get pointer to current frame data
+ */
 const uint8_t* icon_get_data(Icon* icon);
 
+/*
+ * Advance to next frame
+ */
 void icon_next_frame(Icon* icon);