Indicate when trying to assign a 'wrong' shortcut

[Cat-Ion]

In the controllerfixup branch you can't assign keyboard shortcuts in the mouse column, etc. When you try anyway, nothing happens. There should be some visual or audible indicator when a wrong input set is used, like flashing the column header in red or a message.

[fgenesis]

Note to self: Also add a flashing text at the bottom that says "Press any key to assign, Esc to abort, Backspace/Delete/same button to clear" while listening for key input. (Note special case for Menu/Esc key, that one accepts Esc! Check this.)