i3bar: Set `mapped` flag on trayclient creation

Long description:

When a trayclient is first created as a structure in memory, explicitly
set the `mapped` flag to false. Otherwise it may initialize to `true` in
some circumstances without actually being mapped, causing a request to
be mapped from the client to be ignored.

Create the trayclient in memory before handling a request to be mapped
immediately.

fixes #1138

To apply this patch, use:
curl http://cr.i3wm.org/patch/354/raw.patch | git am

@@ -603,17 +603,18 @@ static void handle_client_message(xcb_client_message_event_t* event) {

              * exits/crashes. */

             xcb_change_save_set(xcb_connection, XCB_SET_MODE_INSERT, client);

+            trayclient *tc = smalloc(sizeof(trayclient));

+            tc->win = client;

+            tc->xe_version = xe_version;

+            tc->mapped = false;

+            TAILQ_INSERT_TAIL(output->trayclients, tc, tailq);

+

             if (map_it) {

                 DLOG("Mapping dock client\n");

                 xcb_map_window(xcb_connection, client);

             } else {

                 DLOG("Not mapping dock client yet\n");

             }

-            trayclient *tc = smalloc(sizeof(trayclient));

-            tc->win = client;

-            tc->xe_version = xe_version;

-            TAILQ_INSERT_TAIL(output->trayclients, tc, tailq);

-

             /* Trigger an update to copy the statusline text to the appropriate

              * position */

             configure_trayclients();