Disable KeyBindingManager on right Alt keydown.

This fixes issue #8666.
I think we're safe to remove the public API `setEnabled()` which was added in this pull request #479 as a hack. It was called from Dialogs.js but was later removed by #4110. I checked in Extensions and none except https://github.com/mardraze/brackets-international-keyboards which will be obsolete by this pull request. Or we can put a deprecation warning message for now.