add function key callback

[GoogleCodeExporter]

similar to 
void root_change_cb(m2_rom_void_p new_root, m2_rom_void_p old_root, uint8_t 
change_value);
the function key callback should get the element and the change value
new function key values are also required
M2_KEY_FN1, M2_KEY_FN2
...
maybe this is only supported by some special handlers...

Original issue reported on code.google.com by olikr...@gmail.com on 24 Sep 2012 at 5:19

[GoogleCodeExporter]

Well, I would need something similair.

I have few extra buttons which override completely device functionality.
I would like to be able to register these buttons as quick keys or FN keys and 
be able to use them globally - in all menus. Ideally, I would like to be able 
to get callback when they are used (pressed), so I can change my menu to 
different root ( right now I process it manually outside library loop)

Original comment by jkpub...@kartech.biz on 29 Apr 2014 at 8:27

[GoogleCodeExporter]

well, yes...
As a workaround: You can reuse elements, e.g. a list with some buttons can be 
reused in several menus. So you can use the quick keys for this: Define the 
buttons once (maybe with empty labels) and reuse the buttons on multiple dialog 
boxes. 

Original comment by olikr...@gmail.com on 29 Apr 2014 at 5:13