Use unix-style line endings in script editor on OSX.

Fixes issue [#10654](http://trac.mantidproject.org/mantid/ticket/10654)

Tester: This needs to be verified on OSX. To reproduce the problem before merging:
1. Start MantidPlot
2. Open the script window
3. Type a few separate lines of text and save the file.
4. Close the file in the window and then reopen it again. The line endings will have disppeared!

Merge the fix and try the steps above. It should give you back the file as before.