viewSelector: Remove duplicate call to showPage

We were calling twice showPage() with the correct page, here and in
show() / zoomFromOverview given that _resetShowAppsbutton was called
from the signal 'showing' of overview.  Given that the call to
_resetShowAppsbutton is only actually used when hiding the overview we
can actually put the checked state of the button to false when animating
from overview so it shows the workspace page, causing the same behavior
of _resetShowAppsbutton without all the shenanigans of resetting when
the hiding overview signal is triggered.

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 without having created the thumbnails yet, so reporting a
width of 0 and confusing the slide animation.

Currently it works because gjs calls the callbacks following the order
of the clients connecting that signal, and 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 accessible when requesting the
preferred size.

https://bugzilla.gnome.org/show_bug.cgi?id=732901
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 necessary because of the bug that previous patch fixed,
so now we can just delete that.

https://bugzilla.gnome.org/show_bug.cgi?id=732901
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 other pages
like apps page or search page since the windows come from nowhere
with an initial position not known to 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