Validate keyboard configuration

[lpaulsen93]

I accidentally set "stupid" keys in the keyboard configuration (Change keys menu option) which lead to an undesired behaviour. E.g.:

• setting "F1" for toggling full screen mode made a second window appear on switching to full screen
• setting "RETURN" for toggling full screen mode made keyboard inputs stop working

As I wrote above this was of of course my fault but as an improvement I suggest to validate the keys on Change keys and reject inputs which cannot work properly.