followup bug for ZBX-1177
copying over (possibly) unresolved parts

1. MS Explorer: after drag and drop in Screen Edit screen, one cell goes to edit mode.
2. MS Explorer: Flash clock cannot be dragged and dropped. Please make all cell region draggable! Under Firefox it is not easy to drag the clock.
5. Also, add must support selection of multiple screens. Add->Select multiple Screens->Select, that's it, no need to press Add once again.

in all browsers dragging element but not dropping it on a new cell will result in editing form being open - that should not happen. not dropping on a valid target should not do anything