Make hotkey for "Show/Hide the captured cursor" be configurable

[koppor]

On German keyboards, ` is a "dead letter", which means, one has to press shift + the key for ` and then space. This is not a good hotkey (and it does not work). It would be nice to have this hotkey configurable. I would trying ! as this only involved Shift.

Refs #620

[liulex]

I am afraid I can't make it available right now. I do have a plan to make such keys configurable, but not now, at least not in this year...

[liulex]

Hmm.. If you think ! is acceptable for German keyboards, I may ADD this key to trigger the "Show/Hide the captured cursor" feature. This would be easier.

[deltaidea]

Both ` and ! now toggle cursor visibility, so this is done.