Re #5939. Remove existing ipython console launch mechanism.
Re #5939. Remove usage of existing script interpreter.

Have left the QTextEdit (that was previously only used temporarily)
as a placeholder.
Re #5939. Remove existing script interpreter class.
Re #5939. Add an IPython console as the script interpreter.
Re #5939. Run mantidplotrc on IPython startup.

This is in place of importing the individual mantid & mantidplot
modules and means that code completion in the 'F3 script window'
still works.
Re #5939. Better way of constructing the path to mantidplotrc.py
Re #5939. Make sure any local IPython is picked up first.

Before a system installed one.
Re #5939. Add a mechanism to avoid blocking MantidPlot while running.

We replace the IPython method that actually executes the entered code
with one that then calls the original one on a separate thread so that
we can continue to process GUI events while the command runs.