libweston-desktop: implement the new xdg_shell (stable) protocol

Some clients like the mpv video player now request the xdg_shell
protocol so these will fail if the compositor only provides the
xdg_shell_unstable_v6 protocol. Compositors like mir and gnome provide
both protocols.

The two protocols are very similar therefore the code in xdg-shell-v6.c
has been refactored to work with the new xdg_shell protocol and now
resides in xdg-shell.c.

Pekka:
- split the patch
- fix continued line alignment

Daniel
- allow anchor_rect to initially have zero dimensions
- account for get_popup allowing NULL parent surface

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>