This is a suggestion to support the values 'fullheight and 'fullwidth
for the fullscreen frame parameter on pgtk.
Background: I undecorate the frame when it is maximized or "tiled" (in
Gnome) to the left or right (-> fullheight, currently not supported on
pgtk - only with the X backend).

The patch uses the GDK_WINDOW_STATE_*_TILED and
GDK_WINDOW_STATE_*_RESIZABLE flags that are available since GTK
3.22.23.  A value of 'fullheight is set if the top and the bottom edges
are marked as tiled and not resizable; 'fullwidth is set if the left
and right edges are marked as tiled and not resizable.
So, is the introduced behavior correct? I think it is consistent with
the description in the Emacs manual: "a fullwidth frame is as wide as
possible, a fullheight frame is as tall as possible".

On Gnome, it works as expected. Other desktop environments, such as
KDE, Xfce or tiling window managers don't seem to support the
GTK_WINDOW_STATE_*_{TILED/RESIZABLE} flags, so fullheight and fullwidth
will still never be set there (however, this also seems to be the case
with the Emacs X backend).

Regards,
Flo