Re #7950. Make sure empty list is loaded back correctly.

An empty list of recent files is saved to the QSettings as "@Invalid".
Reading this back in sets the variable to 'None', which causes problems.
Change it back to an empty list.
Re #7950. Remove barrier to starting reduction gui outside MantidPlot.

There are other places with an unguarded import of MantidPlot, but this
one is trivial to fix.
Re #7950. Set PyQt API version for stand-alone running.

If running in Mantid, this setting is done within the C++ code. I'm adding
it here to make sure things work if running outside of MantidPlot (which
doesn't work at the moment, but could cause confusion later if that's fixed).
Note that this has to be done before the first import of PyQt.