added support for _NET_WM_USER_TIME

- add support for _NET_WM_USER_TIME
- add support for _NET_WM_USER_TIME_WINDOW

- now tracks _NET_WM_USER_TIME updates on all clients and compares
  the initial map time to the initial _NET_WM_USER_TIME to determine
  whether to map on focus per wm-spec-1.5.