Let negotiating clients be forced to server-side decoration, and document the GTK half

Asked to research how KDE and GNOME make decorations consistent, after being
told too quickly that srdwm could only control its own titlebar.

Measured against a nested srdwm, one client at a time: Qt/KDE creates a
decoration object and asks for server-side; winit creates one and asks for
CLIENT-side; GTK never creates one at all. The xdg-decoration spec says the
compositor "can decide not to use the client's mode and enforce a different
mode instead" and that the client "must obey" - so the first two are
srdwm's to decide, and it had simply been deferring. The same spec closes
the door on the third: a client that does not negotiate continues to
self-decorate, and GTK is not having the conversation.

New theme.decorations.force_server_side, default off, overrides the client's
requested mode. Verified: with it off Alacritty draws its own content to the
window's top edge, with it on the same window gets srdwm's titlebar --
(0,0,0) versus (46,52,64) sampled at three rows. Off by default because it
cannot move a GTK button and can stack srdwm's titlebar on a client that
draws its own regardless, which is the Firefox case already recorded here.

The GTK half needs no compositor code, only the desktop setting GTK actually
reads - xdg-desktop-portal's org.gnome.desktop.wm.preferences button-layout
for GTK4, gtk-decoration-layout for GTK3. This machine was serving
"close,minimize,maximize:" (left) while srdwm's own button_side was right,
which is the entire mismatch. Documented in DEFAULTS.md with the mapping
from button_side to layout string, and noted that this is exactly what
kde-gtk-config does for KWin.

525 tests pass, clippy clean.

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