Refs #9204 - Allow the MASKFILE command in User Files.

Also show paths in Masking tab of interface.

Now is as good a time as any to slowly introduce the use of a
PropertyManager to share settings between ReductionSteps and between the
C++ and Python layers of Mantid.  See code for further info.
Merge branch 'feature/9204_allow_maskfile_commands_in_sans_isis_user_files' into develop