Add support for _NET_WM_STATE_DEMANDS_ATTENTION.

_NET_WM_STATE_DEMANDS_ATTENTION indicates that some action in or with
the window happened. It's a weaker hint than urgency flag of WM_HINTS,
but some applications and almost all Qt applications use it instead of
WM_HINTS' urgency flag (one example is Skype).

src/ewmh.c

@@ -164,5 +164,5 @@ void ewmh_setup_hints(void) {
     /* I’m not entirely sure if we need to keep _NET_WM_NAME on root. */
     xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_WM_NAME, A_UTF8_STRING, 8, strlen("i3"), "i3");
 
-    xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_SUPPORTED, XCB_ATOM_ATOM, 32, 16, supported_atoms);
+    xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_SUPPORTED, XCB_ATOM_ATOM, 32, 19, supported_atoms);
 }