switcherPopup: Rename destroy() to fadeAndDestroy()

switcherPopup: Rename destroy() to fadeAndDestroy()

In the process of purging all usages of Shell.GenericContainer
of GNOME Shell, one specific problematic situation that might
occur is when classes have functions that would clash with any
ClutterActor or StWidget function name.

One of such example is SwitcherPopup.destroy(). Right now, this
class is a pure JavaScript class that wraps a real actor, but
soon this will change, and it'll become a St.Widget subclass.

Another problem with functions that mimic the toolkit ones is
the predictability of them; after calling destroy(), that widget
is expected to not be available anymore. In SwitcherPopup case,
it is still available for a short while. In this case, that's not
a big problem, but the show() and hide() functions in other clases
are more problematic because the actor's visibility does not
follow that.

This commit is a first step in cleaning that up, and changes the
SwitcherPopup.destroy() to fadeAndDestroy().

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/153