Use the same icon as volume was muted

Show audio-volume-muted-symbolic when volume is 0 just like we do when
muted because for the user it's just the same thing.
Change AppChooser key binding to Super + A

The user can remember this better.

Issue: #63
Map super keys to evdev keys

Super left and right association.
Correct alpha value

Alpha is in 0:1 range.
Shortcuts overlay

List all shortctus in an overlay.

Closes #72
Make volume overlay at least as big as its contents

Properly sized volume overlay.
Size is capped can't exceed screen size.
Fix splash surface with multiple outputs from Xorg

When running weston --output-count=2 (or any number greater than 1) from
X11 results in a black opaque surface that covers the whole desktop.

Weston already has a layer for the fade surface, let's use this instead
of using our own. This also has the side effect that the cursor is no
longer shown on top of the fade surfaces.

We also create one giant surface that covers all the outputs and
destroy it as soon as it's not needed.

This change also make createBlackSurface() public, so that Splash can
have access to it.

Closes #73