desktop-shell: add present() GUI notifications

Each time a shell surface gets mapped in desktop-shell,
an invisible widget resembling a red tooltip, stacked
on the left side on the panel near the clock, is created.

When present() is called on the surface more than once,
the widget becomes visible and the user can click on it
to make the surface appear in the foreground of the
current workspace.

The widget is then destroyed. It is also destroyed if the
corresponding shell surface gets destroyed by any means.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
Signed-off-by: root <root@debian70>