Wayland: Add GLFW_WAYLAND_APP_ID window hint

This adds a window hint string for the xdg_toplevel::app_id, which is
used by desktop environments to connect windows with application icons
and other information.  This is similar to the WM_CLASS property on X11.

A few very minor fixes were done by @elmindreda during merge.

Fixes #2121
Closes #2122

src/internal.h

@@ -421,6 +421,9 @@ struct _GLFWwndconfig
     struct {
         GLFWbool  keymenu;
     } win32;
+    struct {
+        char      appId[256];
+    } wl;
 };
 
 // Context configuration