dnd: Make startDrag() fail if there is a current grab

dnd: Make startDrag() fail if there is a current grab

This call just went through stomping over previous drag operations if any,
_maybeStartDrag() accounted for this, but other callers (well, WindowClone
in workspace.js) don't. This must bail out early even if a drag operation is
requested, luckily all callers account for it already.

This broke shell state by preserving connected captured-event handlers if
one tried to drag multiple windows simultaneously through multitouch. We
of course don't support that, now more elegantly.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/455