desktop::handle mult-screen and desktop workarea changed by dock

solve:
1.allways show desktop in primary screen
2.only primary screen can use
3.the others screen only have background , but cannot use

problem:
1.when click the other screens,the primary screen desktop will hide.
Debug with windowunmap the 'DDE BACKGROUND',and then the desktop is ok.
2.use dock_height instead of dock_panel_height

solve:
different dock display mode has different dock_region_height,
    so the desktop should update_workarea_size when dock display mode changed

tower:https://tower.im/projects/8a6342c556be4c4eb898fdd7fd6fcb2f/todos/73c6c2e7ac564b20b5552a048a095000/

Change-Id: I9810c766a53aea34354ba274c73d49fbad3540d8