clients/window: Add viewport destination support

clients/window: Add viewport destination support

Add support for setting the widget's destination wp viewport.
Setting it in the widget instead of being set directly by the client
ensure that the widget can be identified in widget_find_widget.

v2: Return -1 on error (Pekka)
    Scale allocated x and y when viewport is set (Pekka)
    Allow user to set -1 for viewport width and height (Pekka)

v3: Use NULL instead of 0 (Daniel)
    return 0 if width and height are -1 (Daniel)

Signed-off-by: Harish Krupo <harish.krupo.kps@intel.com>