Re #5151. Drop the test until I can figure out the symbol export

for Windows. If I can.
Merge branch 'feature/5151_remove_custominterfaces_dependencies' of https://github.com/mantidproject/mantid into feature/5151_remove_custominterfaces_dependencies
Re #5151. Add back the unit test.

And add a DllConfig file to deal with the symbol import/export on Windows
so that the test can link to it. Note that there's a QT macro that's used
for this here rather than our own DllImport/Export ones.