fix(developer): handle LControl being set by Windows when AltGr pressed

Fixes #7506.

In the keyboard debugger, we need to mask out LControl when AltGr is
pressed by the user, because Windows will generate LControl + AltGr for
European layouts.

Note: have also hidden the System Keyboard debug setting in this commit,
given it is not currently implemented; see #1225.

A follow-up issue is #7529.