main: Rename widget to match reality

We're not checking whether the volume button is visible, but if the
volume button's popover is visible.
backend: Only schedule hiding the popup if we're showing it

We don't need a timeout to hide the popup if we're hiding the popup
already.

https://bugzilla.gnome.org/show_bug.cgi?id=736439
backend: Don't always hide popup when cursor leaves window

But only if the popup isn't busy. Otherwise we end up overriding
the "mark busy" inhibitor and we'd be in a weird state.

https://bugzilla.gnome.org/show_bug.cgi?id=736439
backend: Always cancel popup timeout when marking popup as busy

So as to avoid the popup hiding when it's busy.

https://bugzilla.gnome.org/show_bug.cgi?id=736439