[x11] Add support for _GTK_FRAME_EXTENTS

Summary:
KDE is known for having a strong view on the client-side decorations vs
server-side decorations issue. The main argument raised against CSD is
that desktop will look less consistent when clients start drawing window
decorations by themselves, which is somewhat true. It all ties to how
well each toolkit is integrated with the desktop environment.

KDE doesn't control the desktop market on Linux. Another big "player"
is GNOME. Both KDE and GNOME have very polarized views on in which
direction desktop should move forward. The KDE community is pushing more
toward server-side decorations while the GNOME community is pushing
more toward client-side decorations. Both communities have developed
great applications and it's not rare to see a GNOME application being
used in KDE Plasma. The only problem is that these different views are
not left behind the curtain and our users pay the price. Resizing GTK
clients in Plasma became practically impossible due to resize b...