Allow controls to override hotkey sequences

[petchema]

Disable hotkey sequences when controls with overridesHotkeySequences flag have the focus

Part of fix for issue #1707

[petchema]

That could be made even a bit more flexible if controls could provide some keyboard events handler, returning true if they handled the event on their own; That way one could implement partial overrides.
I don't know if we'll require the extra flexibility, but it doesn't seem too difficult to implement.
On the other hand it may feel "too little" a change, given that most keyboard handling is mixed with other concerns in different Update() methods...
So for the moment my position is that this "simplest change that works" is sufficient.