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/news.dox

@@ -37,6 +37,16 @@ SDK](https://vulkan.lunarg.com/).
 For more information see @ref vulkan_guide.
 
 
+@subsubsection wayland_libdecor_33 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 content_scale_33 Content scale queries for DPI-aware rendering
 
 GLFW now provides content scales for windows and monitors, i.e. the ratio
@@ -443,6 +453,9 @@ Use Wayland or X11 instead.
  - @ref GLFWwindowmaximizefun
  - @ref GLFWwindowcontentscalefun
  - @ref GLFWgamepadstate
+ - @ref GLFW_WAYLAND_LIBDECOR
+ - @ref GLFW_WAYLAND_PREFER_LIBDECOR
+ - @ref GLFW_WAYLAND_DISABLE_LIBDECOR
 
 
 @subsubsection constants_33 New constants in version 3.3