Support multiple directories in the art_directory property

[KeyWorksRW]

Description:

Currently, the art_directory can only hold a single path. That's probably fine for a small project, but larger projects could easily have more then one location for artwork. We're going to run into that ourselves -- once wxUiTesting is added, it will need both it's own art directory as well as the art_src folder that's part of the main wxUiEditor source code.

With some work, we should be able to use the Grid dialog that supports multiple fields to add multiple paths -- while we only need one column, it needs to be a control that allows the user to search for a path.

[Randalphwa]

I think a better solution would be to add an override art directory property to folders. That avoids the possible issue of identical filenames with only the extension changing, or for the user to know exactly where the art file came from. Keep in mind that the user isn't limited to the art directory for images -- it just makes it easier to browse for the art file the user wants.