Bugfix: Eliminate race condition, fix dock windows

There was a race condition when mapping a window and not setting the event mask
before. Therefore, the ReparentNotify and (more important) the UnmapNotify generated
by reparenting were not received, thus leaving the awaiting_useless_unmap variable
of the client "true". To just make it work, in previous commits the DestroyNotify
handler was introduced. Fortunately, with fixing this race condition by first
setting the event mask and mapping the window afterwards, we can remove this handler.

As for the dock windows, there were quite some occurences were client->container
was used without checking if the client is inside a container at all.

Furthermore, the client’s strut containing the space to reserve at the screen edge
is now checked and the desired height is set to the window’s height if the strut
contains 0 or if no strut was specified at all.

include/handlers.h

@@ -18,7 +18,6 @@ int handle_button_press(void *ignored, xcb_connection_t *conn, xcb_button_press_
 int handle_map_request(void *prophs, xcb_connection_t *conn, xcb_map_request_event_t *event);
 int handle_configure_event(void *prophs, xcb_connection_t *conn, xcb_configure_notify_event_t *event);
 int handle_configure_request(void *prophs, xcb_connection_t *conn, xcb_configure_request_event_t *event);
-int handle_destroy_notify_event(void *data, xcb_connection_t *conn, xcb_destroy_notify_event_t *event);
 int handle_unmap_notify_event(void *data, xcb_connection_t *conn, xcb_unmap_notify_event_t *event);
 int handle_windowname_change(void *data, xcb_connection_t *conn, uint8_t state,
                                 xcb_window_t window, xcb_atom_t atom, xcb_get_property_reply_t *prop);