dnd: Implement touch event support

dnd: Implement touch event support

We need to keep track of the pointer emulating touch sequence. In order
to have events properly redirected on touch devices, the
Clutter.grab_pointer and ungrab_pointer() have been replaced by the grab()
and grab_sequence() ClutterInputDevice methods, one or the other is used
depending on the device triggering DnD.

An extra "sequence" argument has been added to startDrag, passing null here
will resort to pointer grabs.

This is enough to make thumbnails in the WorkspaceBox draggable through
touch.

https://bugzilla.gnome.org/show_bug.cgi?id=756748