system: Emulate click action button release

Since commit 2c070d38, we add a ClickAction to the visible AltSwitcher
button to track long-presses. As a result, we now have two components
that will grab and ungrab the pointer for the button, so to make sure
we don't end up with a stuck grab, we need to release the second's
component grab when the first activates.

Currently we only drop the StButton grab on long-press, we also need
to cancel any initiated long-press on click.

https://bugzilla.gnome.org/show_bug.cgi?id=781738