ivi-shell: unmap views which are not in scenegraph

From Michael Olbrich:
"Both the core in weston_compositor_build_view_list() with view.link
and the ivi-shell in commit_screen_list() with view.layer_link
don't remove the old views from the list.

As a result, all views that are not currently in the list have
old broken links. Destroying such a view tries to remove
the view from these lists and will access the old, invalid pointers."

Therefore, we have to unmap weston_views which are not in current
scenegraph of ivi-shell. I implemented ivi_view_is_mapped() function
to check mappedness of ivi_views. The functions checks:
   - the view is on a layer's order list
   - the layer is on a screen
   - the layer and view's ivi_surface are visible

If ivi_view is not mapped but weston_view is still mapped,
we have to unmap the weston_view with weston_view_unmap() call.

Reported-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>