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.
DashSlideContainer: add initial slide parameter.
Track only the  input region  of DashSlideContainer.

Probably after a rebase, I end up being tracking the inputregion og the _box
and dash._box actors instead of that of the slide, that exists exactly to be
tracked. This makes the area behind the hidden dock when placed on a right
monitor finally  unclickable.
Merge branch 'experimental_DashSlideContainer' into experimental

Conflicts:
	dockedDash.js