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.

include/GLFW/glfw3.h

@@ -1063,6 +1063,9 @@ extern "C" {
 #define GLFW_EGL_CONTEXT_API        0x00036002
 #define GLFW_OSMESA_CONTEXT_API     0x00036003
 
+#define GLFW_WAYLAND_PREFER_LIBDECOR    0x00038001
+#define GLFW_WAYLAND_DISABLE_LIBDECOR   0x00038002
+
 /*! @defgroup shapes Standard cursor shapes
  *  @brief Standard system cursor shapes.
  *
@@ -1123,6 +1126,11 @@ extern "C" {
  *  macOS specific [init hint](@ref GLFW_COCOA_MENUBAR_hint).
  */
 #define GLFW_COCOA_MENUBAR          0x00051002
+/*! @brief Wayland specific init hint.
+ *
+ *  Wayland specific [init hint](@ref GLFW_WAYLAND_LIBDECOR_hint).
+ */
+#define GLFW_WAYLAND_LIBDECOR       0x00053001
 /*! @} */
 
 #define GLFW_DONT_CARE              -1