e_client: add feature - show_pending

e_client: add feature - show_pending

E supports show_pending feature.
If an ec set show_pending, then e doesn't show it's frame, doesn't calculate visiblity
and doesn't send visibility change event until unsetting show_pending.

- e_client_show_pending_set
- e_client_show_pending_unset

FYI,
if an ec which is running show_pending requests unmap, then e set ec's visibility to
UNOBSCURED. By this, its visibility can be changed to FULLY-OBSCURED by calculating
visibility.

Change-Id: I3fd89b706ac2d93c01248e988d271ac77f03ecb6