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

include/GLFW/glfw3.h

@@ -1164,6 +1164,9 @@ extern "C" {
 #define GLFW_ANGLE_PLATFORM_TYPE_VULKAN  0x00037007
 #define GLFW_ANGLE_PLATFORM_TYPE_METAL   0x00037008
 
+#define GLFW_WAYLAND_PREFER_LIBDECOR    0x00038001
+#define GLFW_WAYLAND_DISABLE_LIBDECOR   0x00038002
+
 #define GLFW_ANY_POSITION           0x80000000
 
 /*! @defgroup shapes Standard cursor shapes
@@ -1307,6 +1310,11 @@ extern "C" {
  *  X11 specific [init hint](@ref GLFW_X11_XCB_VULKAN_SURFACE_hint).
  */
 #define GLFW_X11_XCB_VULKAN_SURFACE 0x00052001
+/*! @brief Wayland specific init hint.
+ *
+ *  Wayland specific [init hint](@ref GLFW_WAYLAND_LIBDECOR_hint).
+ */
+#define GLFW_WAYLAND_LIBDECOR       0x00053001
 /*! @} */
 
 /*! @addtogroup init