Don't allow capture of "system" keys

[martinthomson]

Mozilla's position is that reservation of specific keys is better managed by the browser (or maybe web extensions). For instance, an "installed" web app (whatever that means) might be able to capture more key combinations than a random site. This might allow something like a remote desktop app to be installed and then capture Alt-Tab (or equivalent). Or, this could be as simple as having a browser extension manage a list of sites for which the Windows key is disabled when they are fullscreen.