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.
Refs #9204 - Fix RHEL6 system test.

New `settings` object clashed with old `settings` method.
Refs #9204 - Make sure new settings object is cleared ...

... whenever the singleton is reset.

Also, slight change so that we only ever load the instrument once on
interface startup.
Refs #9204 - Workaround for bug in MaskDetectors ...

... so that things are no longer "flipped".
Refs #9204 - Explicitly raise "user file loaded" flag after startup.
Merge branch 'master' into feature/9204_allow_maskfile_commands_in_sans_isis_user_files

Conflicts:
	Code/Mantid/scripts/SANS/isis_reduction_steps.py

Refs #9204.
Merge remote-tracking branch 'origin/feature/9204_allow_maskfile_commands_in_sans_isis_user_files'