Improve the script path processing.

[schmidtw]

This change adds the ability to point to a script file that includes
other script files the same as the script library that comes with QCAD.
The included files are included based on the following search order:

1. The initial script's path
2. The local script directory path (platform dependent)
3. The script directory next to the executable
4. The built in script files via a library

To ensure consistency on possibly overwritten files in different
locations, the list duplication removal function was replaced with a
version that maintains the order of the list.