Use gjs classes.

Just to update to the more recent syntax... no benefits I think.
Add DashSlideContainer Actor.

A simple Actor with one child whose allocation takes into account the slide out
of its child via the _slidex parameter ([0:1]).

This is required since I want to track the input region of this container which
is based on its allocation even if the child overlows the parent actor.

This will solve the problem with multimonitor where the slided out part of the
dash even if invisible due to the clipping still steals input events. Hopefully
it will also help to simplify the code a bit.
Remove code controlling inputMode.

inputModes have been removed, the thing is now somehow automatic:
https://bugzilla.gnome.org/show_bug.cgi?id=700735

This code was also causing a bug preventing the accessibility from working.