overviewControls: Fix typos in comment
overviewControls: Don't update translation on slideIn()

The comment is right, updating the translation should be deferred
to pageEmpty, or else overview controls will just pop up instead
of sliding in. So revert that part of commit 6a7fa52879d53.
overviewControls: Fix initial slide-in animation

Controls are slided in by animating translationX from the actor's
width to 0; however as _updateTranslation() will skip the animation
when the property is already at its target value and 0 happens to be
the initial value of translationX, the initial animation is skipped.
Fix this by initializing translationX to undefined, which will always
differ from a valid target value.
overviewControls: Use correct translation in dragBegin

Commit 12845f0eefa reset translationX to 0 to fix the slide-in
animation of the dash when dragging a search result, but 0 is the
wrong value for the thumbnail slider - the next time it will be
shown, it will just pop up instead of sliding in due to translationX
already being at its target value.
Fix this by making _updateTranslation take the actual visibility into
account and call that to set the correct translation in dragBegin.