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

docs/news.dox

@@ -43,6 +43,16 @@ to whatever window is behind it.  This can also be changed after window
 creation with the matching [window attribute](@ref GLFW_MOUSE_PASSTHROUGH_attrib).
 
 
+@subsubsection wayland_libdecor_34 Wayland libdecor decorations
+
+GLFW now supports improved fallback window decorations via
+[libdecor](https://gitlab.freedesktop.org/libdecor/libdecor).
+
+Support for libdecor can be toggled before GLFW is initialized with the
+[GLFW_WAYLAND_LIBDECOR](@ref GLFW_WAYLAND_LIBDECOR_hint) init hint.  It is
+enabled by default.
+
+
 @subsubsection wayland_app_id_34 Wayland app_id specification
 
 GLFW now supports specifying the app_id for a Wayland window using the
@@ -267,6 +277,9 @@ then GLFW will fail to initialize.
  - @ref GLFW_POSITION_X
  - @ref GLFW_POSITION_Y
  - @ref GLFW_ANY_POSITION
+ - @ref GLFW_WAYLAND_LIBDECOR
+ - @ref GLFW_WAYLAND_PREFER_LIBDECOR
+ - @ref GLFW_WAYLAND_DISABLE_LIBDECOR
 
 
 @section news_archive Release notes for earlier versions