desktop:fixed some bug about dock hide-mode and display-mode

1.fixed bug that:when dock hide-mode changed ,but the desktop workarea donot change to right size

2.fixed bug that:when dock display-mode changed ,but the computer and
trash icons not show or hide rightly

3.add some function and signal for desktop-mutl-screen handling

tower:https://tower.im/projects/8a6342c556be4c4eb898fdd7fd6fcb2f/todos/bcee35721ec0455daa4c65dac6715895/
tower:https://tower.im/projects/8a6342c556be4c4eb898fdd7fd6fcb2f/todos/935ec39eca2449e5966802bcc03b2d3b/

Change-Id: I624dd5987c8432ecff0db8c58dc536b178be17c5