Adds a boolean "dragging" property to the EAttachmentView interface,
which becomes TRUE when the user start a drag from the attachment view.
e_attachment_view_drag_motion() and e_attachment_view_drag_drop() both
return FALSE when this property is set.
Also, do not register the entire EMsgComposer window as a drag
destination. Just intercept drag signals from the GtkHTML widget.
Requires gtkhtml commit 344eb5e to fully work correctly.
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