Integrate sysmonitor widget

Fixes trac issue [http://trac.mantidproject.org/mantid/ticket/9843](http://trac.mantidproject.org/mantid/ticket/9843)

Tester: Start MantidPlot and if psutil can be found (on Windows/Mac it is bundled so that should always be true) a system monitor tab is displayed in MantidPlot.