RE 6396 MantidEV now does work outside Qt thread.

This update makes two changes to the initial version of MantidEV
that was previously checked in (before the new git workflow).
1. The first tab now supports loading and converting to MD directly
   from an event file.  This is much more convenient for the typical
   user than separately loading and converting to MD using the
   MantidPlot GUI and just specifying the workspaces to MantidEV.
   The user can still specify and use event and MD workspaces already
   in MantidPlot, if they choose to do so.
2. Running longer calculations like loading, converting to MD, finding
   peaks and integrating within the Qt GUI thread caused the MantidPlot
   GUI to become non-responsive.  To avoid this, a local thread pool
   with one thread was added to MantidEV, and the compute intensive
   tasks are now run in the thread from the thread pool.  Only one
   thread was provided, since the operations typically must be carried
   out in sequence.  Other operations are blocked while the computation
   thread runs.  Since the algorithms that are launched from the one
   thread can in turn launch multiple other threads, using one thread
   at this point does not hurt performance, but will prevent users
   from making simple mistakes, such as trying to find peaks, before
   the MD workspace has been completely constructed.

refs #6396
Merge remote branch 'origin/feature/6396_basic_MantidEV'