Refactor OSD element display code

Remove the giant `select` block that contained all the code to generate the elements and transition them to individual functions called only when the element is active. Simplifies the code and results in a performance improvement as it's not necessary to fall through the large `select` statement for every element that will be drawn. The individual functions and the element to function mapping are moved to a new `osd_elements.c` file.

Moved the OSD related code files to a new `osd/` directory.

Also pre-analyze the active elements and only process those that are active. This also saves processing as it's not necessary to loop through all 50 or so elements when only a couple are active.

Various other cleanup and removal of stale or unnecessary code.

In the default configuration the element drawing phase of the OSD task is reduced from ~51us to ~35us - resulting in about a 30% decrease in processing time.

src/main/cms/cms.c

@@ -65,9 +65,10 @@
 #include "flight/mixer.h"
 
 // For VISIBLE*
-#include "io/osd.h"
 #include "io/rcdevice_cam.h"
 
+#include "osd/osd.h"
+
 #include "rx/rx.h"
 
 #ifdef USE_USB_CDC_HID