xdg-shell: clarify parent requirements for grabbing popups
Closes: #82 Signed-off-by: Kirill Primak vyivel@eclair.cafe
Note: currently, Weston and Smithay-based compositors send xdg_wm_base.error.not_the_topmost_popup
to clients which try to create a grabbing xdg_popup with a non-grabbing xdg_popup as a parent.