Improve path handling in Python scripts directory dialog

[martyngigg]

This issue was originally TRAC 9806

Opened from http://trac.mantidproject.org/mantid/ticket/9792.

This concerns the second tab of the "Manager User Directories" dialog.

There are some improvements to be made around how it handles paths.

If you add a single new path and then click okay you will get a whole duplicate set of directories in sys.path rather than just the new one.

If you remove a path then you have to restart MantidPlot for it to take affect.

Updates to sys.path should be handled better than this.

[martyngigg]

@NickDraper (2014-07-07T09:51:29):
This probably covers how the script repository works as well.

---

@NickDraper (2014-12-08T10:25:34):
Moved to the backlog at the code freeze of R3.3

[martyngigg]

This no longer appears to be in issue