Extend drop support with variant values. A drop may be a file drop or a
value drop or both.

Rename Utils::FileDropSupport to Utils::DropSupport and add methods to
add not only files but any QVariant value to the mime data. Project
explorer adds dragged nodes (which will be needed for future ModelEditor
plugin).

Change-Id: I799542c60fdecb3e64af0d3ba47b6caa9adbcfd7
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>