X11: Clean up EWMH feature detection

The EWMH feature detection atoms are now named and loaded the same way
as other X11 atoms.  Detection is now performed after all
non-conditional atoms have been loaded.  The EWMH detection now has
hopefully more readable comments.

(cherry picked from commit fad9896d38)

src/x11_platform.h

@@ -239,6 +239,8 @@ typedef struct _GLFWlibraryX11
     _GLFWwindow*    disabledCursorWindow;
 
     // Window manager atoms
+    Atom            NET_SUPPORTED;
+    Atom            NET_SUPPORTING_WM_CHECK;
     Atom            WM_PROTOCOLS;
     Atom            WM_STATE;
     Atom            WM_DELETE_WINDOW;