xdg-shell: Make xdg_popup non-grabbing by default

Turn xdg_popup into plain temporary child surfaces without any grabbing
or mapping order requirements by default.

In order to create grabbing popup chains, a new request 'grab' is
introduced which enables more or less the same semantics and
requirements as xdg_popup previously had related to grabbing, stacking
and mapping order.

This enables using xdg_popup for creating tooltips and other user
interface elements that does not want to take an explicit grab.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Signed-off-by: Mike Blumenkrantz <zmike@samsung.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Quentin Glidic <sardemff7+git@sardemff7.net>