xwm: Perform a roundtrip to send a deferred WM_TAKE_FOCUS

WM_TAKE_FOCUS requires a valid timestamp that isn't XCB_TIME_CURRENT. To
get one, we set a property on the window and wait for the notification
that it was set - this notification comes with a valid timestamp.

Once we have that timestamp, delete the property, and fire off the slightly
delayed WM_TAKE_FOCUS client request.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

shared/xcb-xwayland.c

@@ -139,6 +139,7 @@ x11_get_atoms(xcb_connection_t *connection, struct atom_x11 *atom)
                 { "XdndActionCopy",     		F(xdnd_action_copy) },
                 { "_XWAYLAND_ALLOW_COMMITS",    	F(allow_commits) },
 		{ "WL_SURFACE_ID",			F(wl_surface_id) },
+		{ "_WESTON_FOCUS_PING",			F(weston_focus_ping) },
 	};
 
 	xcb_intern_atom_cookie_t cookies[ARRAY_LENGTH(atoms)];

shared/xcb-xwayland.h

@@ -97,6 +97,7 @@ struct atom_x11 {
 	xcb_atom_t		 xdnd_action_copy;
 	xcb_atom_t		 wl_surface_id;
 	xcb_atom_t		 allow_commits;
+	xcb_atom_t		 weston_focus_ping;
 };
 
 const char *