viewSelector: Remove duplicate call to showPage

We were caling twice showPage() with the correct
page, here and in show() / animateFromOverview given
that _resetShowAppsbutton was called from the signal
'showing' of overview.
Also, this call here doesn't belong at all to reseting
the button.

https://bugzilla.gnome.org/show_bug.cgi?id=732901
workspaceThumbnails: allow requesting size at any time

The slide of thumbnailWorkspace is shown when entering overview, connecting to
the same signal that creates the thumbnails, the showing signal of overview,
but, to make the slide animation we need to know how much width the slider has.
To do that we ask the thumbnailsWorkspace about its width, but given that it
connects to the same signal it could ask the width withouth having created the
thumbnails yet, so reporting a width of 0 and confusing the slide animation.

Currently it acccidentaly works because gjs calls the callbacks following the
order of the clients connecting that signal, and accidentally the
thumbnailsWorskpace is connected before the slide ones.

To avoid that we allow to request the preferred size of the thumbnailsBox at any
time with any number of thumbnails. The only thing required is to make sure the
porthole is accesible when requesting the preferred size.
overviewControls: don't override explicit calls to slideIn

Currently we are overriding the explicit calls to slideIn
given that it's called also with the signal of showing overview.

It was necesary because of the bug that previous patch fixed,
so now we can just delete that.
workspace: Fade in instead of zoom to return desktop

The zooming animation of the windows looks nice when animating
from the workspace display page, but looks weird from others pages
like apps page or search page since the windows comes from nowhere
with a initial position not known for the user.

Instead of that just fade the desktop with the windows in its
original position.

https://bugzilla.gnome.org/show_bug.cgi?id=732901