Don't pass any other keys than arrows during volume adjustment

[ivnc]

A Spanish-speaking user has complained through Twitter about the fact that, if you press any other key than arrows during volume adjustment such as Enter, this is passed to NVDA anyway.

Steps to reproduce (case example)

This is the user's particular scenario, a completely valid example to demonstrate this.

1. He focused the NVDA shortcut on Desktop.
2. He started volume setting by activating the add-on command layer.
3. Then he pressed Enter, by custom, to confirm the changes he had just made.
   I've tested this with this repository's webpage in Firefox and one-letter navigation commands are also passed to NVDA, what suggests a general behavior.

Current behavior

Key presses are passed to NVDA despite being on the add-on command layer.

Suggested behavior

Block all other keys different from arrows and the keystroke to disable the add-on layer. Either notify the user or automatically exit the layer if one of these is pressed.

[yplassiard]

Accepted

This is indeed something which may be confusing at some point. I will figure out how I can either block key presses, or indicate that we exited the adjust volume layer.