xdg_shell: make view floating if a parent has been set

Prompts e.g. authentication request from firefox-wayland ought to be
floating.

This is a bit coarse but just fixed size is not enough, here is what
firefox does:
[1285461.363]  -> [email protected]_xdg_surface(new id xdg_surface@68, wl_surface@71)
[1285461.508]  -> [email protected]_toplevel(new id xdg_toplevel@67)
[1285461.571]  -> [email protected]_parent(xdg_toplevel@37)
[1285461.630]  -> [email protected]_title("Authentication Required")
[1285461.736]  -> [email protected]_app_id("firefox")
...
[1285476.549] [email protected](0, 0, array)
...
[1285502.080]  -> [email protected]_min_size(299, 187)
[1285502.140]  -> [email protected]_max_size(1920, 32767)

This can also be observed with e.g. the open window of gedit
(gedit->open->other documents)

0 files changed, 0 insertions, 0 deletions