Simplify how we handle the drag and drop. Now the drop is always handled
by the EContentEditor first and then if the data are processed by the
editor then it's handed to the EMsgComposer. Also the DOM operations
are now handled on the web extension side to save some DBus calls.

Correctly handle the history if the drop operation is copying the content
and not moving it.

Fix the history if we are dragging a content where there is a space
before the selection start. WebKit will remove it unless there is a
space after the selection end.

Also there is a weird thing going on and I still don't know if it's caused
by WebKit or Evolution. If dragging content around the editor sometimes the
current selection is changed. The problem is that if moving the content,
then WebKit is removing the currently selected content and at that point it
could be a different one from the dragged one. So before the drop is
performed we restore the selection to the state when the drag was initiated.