Override WorkspaceSwitcherPopup._show() instead of the whole object

This makes the code a bit simpler, but just as effective. This was
tested on 3.12, but review of the gnome-shell code indicates it should
also work on 3.0 and probably everything in between as well.

By overriding _show (as opposed to display), this extension now plays
well with the Frippery Bottom Panel extension, which provides its own
(horizontal) workspace switcher popup. Since it still uses the original
_show function, with this change the Frippery version of the switcher is
also hidden as well when both extensions are combined.

A small downside of overriding _show instead of display is that the
switcher popup is still created and, after a timeout, destroyed in the
background, it's just not being shown. This should not cause any
problems, though.