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.
DashSlideContainer: set width when fully slideout to 1px.
DashSliderContainer: add paramater for slide direction.
Use DashSliderContainer. Code cleanup.

Start to use the new DashSliderContainer. This should solve the bug concernig
stolen clicks with multimonitor-configuration and primary monitor on right. The
code has been partially cleanup with reorganization of the dash constructor.
Some code is necessary anymore with the new container such as the clipping
function and the Clutter.Gravity machinery and thus has been removed.
Remove Bolt extensin compatibility support code.

As the Bolt extensions is not going to be updated for new version of Gnome
Shell, I'm dropping the compaitibility code for it.