overviewControls: Don't allow appearing controls to "pop in"

When coming back from search or apps, the workspace thumbnails and dash
don't slide in but "pop in". This is because of bad timing: when slideIn
is called, we immediately start the translation animation, and it
completes before by the time we fade the new page in.

Fix this by calling slideIn and slideOut at two different times: we now
slide out when the old page with our controls is fading out, and slide in
when the new page with our controls is fading in.

https://bugzilla.gnome.org/show_bug.cgi?id=708340