Gnome 3.8 compatibility: show/hide DashSlide's actor instead of dash's one.

For some reason hiding and showing () the dash actor makes the parent container
of the default dash be wrongly placed in the middle of the overview.
Showing/hiding it instead seems to work properly.
Gnome 3.8 compatibility: manually set input mode on dash accessibility

The function handling the accessibility callback (CtrlAltTabManager.focusGroup)
slightly changed. Update my code accordingly.
Do not exit from accessibility automatically dropping basic keypress handling.

Following the upstream changes, there is now a windows group to focus back on
the windows, thus remove the keypress handling and all now unused helping
functions.

This is an accessibility function while when I wrote this code I had in mind
the idea of adding keyboard shortcuts. I will add back again similar code if I
will consider to add keyboard shortcuts. Note that the  isMenuUp variables I
was monitoring to keep the dash visible was removed from upstream and a new
trick will have to be found.