Brightness media keys support

[azmeuk]

Thank you for this great extension. Some keyboards have media keys dedicated to screen brightness. The shortcut selection dialog did not react on these key pressing so I could not bind the brightness to these keys.

[Igetin]

I think this is basically a duplicate of issue #109. Your keys should work if you use a modifier key with them. For example, hold Alt and then press the key:

The problem is that the shortcut editor does not seem to allow single-key shortcuts at the moment.

[azmeuk]

Nice catch.