for selecting multiple cells

1. moving cursor on cell's hyperlink doesn't work
2. clicking to linked page doesn't work

side effect
http://tracker.zkoss.org/browse/ZSS-864

issue detail:
when we're in none side effect situation
1. AreaCtrl._doMouseMove will be triggered to change cursor and set hyperlink element when mouse cursor in moving on multiple cells selection.
2. should trigger ssheetCtrl._doMousedown. but side effect case didn't.