Wayland: Add initial support for libdecor

This is partly based on the implementation of libdecor support in
PR #1693 by @ christianrauch.

Where available, the libdecor library is loaded at init and becomes the
preferred method for window decorations.  On compositors that support
XDG decorations, libdecor in turn uses those.  If not, libdecor has
a plug-in archtecture and may load additional libraries to either use
compositor-specific decorations or draw its own.

If necessary, support for libdecor can be disabled with the
GLFW_WAYLAND_LIBDECOR init hint.  This is mostly in case some part of
the dynamic loading or duplication of header material added here turns
out to cause problems with future versions of libdecor-0.so.0.

Fixes #1639
Closes #1693
Related to #1725

This was adapted to 3.3-stable from
fbdb53b9ca.

docs/compat.dox

@@ -122,6 +122,14 @@ wayland-protocols 1.6, and mandatory at build time.  If the running compositor
 does not support this protocol, the screensaver may start even for full screen
 windows.
 
+GLFW uses the [libdecor library](https://gitlab.freedesktop.org/libdecor/libdecor)
+for window decorations, where available.  This in turn provides good quality
+client-side decorations (drawn by the application) on desktop systems that do
+not support server-side decorations (drawn by the window manager).  On systems
+that do not provide either libdecor or xdg-decoration, very basic window
+decorations are provided.  These do not include the window title or any caption
+buttons.
+
 GLFW uses the [xdg-decoration
 protocol](https://cgit.freedesktop.org/wayland/wayland-protocols/tree/unstable/xdg-decoration/xdg-decoration-unstable-v1.xml)
 to request decorations to be drawn around its windows.  This protocol is part