Improve threading model for Python scripts

[Nick Draper]

Ideally we want the execution of the script to occur in it's own thread. You will have to catch every interaction with the user interface and marshall the call onto the correct thread.

Also while doing this it is worth thinking about what we may be able to do in terms of progress reporting for long running scripts. Is there any way to have an execution pointer point to the line of the script being executed?

[Martyn Gigg]

(In [1931]) Scripts are now executed so that the Mantid algorithms are run in a separate thread and the user can continue interacting with MantidPlot. Re #359

[Martyn Gigg]

(In [1973]) Fixed the progress arrow for script execution in Windows, I didn't realise that it wasn't working there. Re #359

[Martyn Gigg]

(In [1974]) Slight fix to progress arrow in script window when only running a selection of the code. Re #359

[Martyn Gigg]

(In [1977]) Added an abort script action to the script window context menu and made sure that the application asks before closing while still running a script. Also the algorithm dialog now monitors things correctly. Re #359

[Martyn Gigg]

(In [1983]) Added some output to script window indicating when a script is started and whether it finishes successfully. Also made better use of the arguments for the Python sys.exit command. Re #359

[Martyn Gigg]

(In [1984]) A slight fix to the tab indentation of the simple Python API to prevent some algorithms being run twice in a script. Re #359

[Stuart Campbell]

This ticket has been transferred to github issue ​1207