Wayland: Use tags to verify proxy ownership

This is in preparation for adding support for libdecor, which creates
its own proxies on our display.  It will likely also be helpful to some
people using native access on Wayland.

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

(cherry picked from commit 91c837ace5)

src/wl_platform.h

@@ -257,6 +257,8 @@ typedef struct _GLFWlibraryWayland
     _GLFWwindow*                dragFocus;
     uint32_t                    dragSerial;
 
+    const char*                 tag;
+
     struct wl_cursor_theme*     cursorTheme;
     struct wl_cursor_theme*     cursorThemeHiDPI;
     struct wl_surface*          cursorSurface;