Add first cut at integrating S. Millers' system monitor.

At the moment it requires psutil to work.
Refs #9843
Add a check for the psutil package

The system monitor is only made available if this package is present.
Refs #9843
Merge branch 'feature/9843_integrate_sysmonitor_widget' into develop

Conflicts:
	Code/Mantid/MantidPlot/src/ApplicationWindow.h