Add folder view to structure select/save GUIs

[shadowmage45]

Problem: When a person has many structures in their included folder, it becomes hard to organize and view them, as they are all in one long list.

Feature: Include a basic file-selection GUI for structure selection and saving -- can borrow code/basis from MEIM (as that is how the model/texture/etc files are handled there).

Will need some way to easily differentiate between files and directories, as well as a way to lock it from non-server OPs. (file system access should not really be allowed from MC)

Will need basic navigation aids -- e.g. selecting previous directory, or a single directory from the current path. Will need to bound directories to ensure they cannot leave the current setup/installed area. Perhaps pre-scanning and validating what folders to display?

[shadowmage45]

I have decided that file system access, even basic access such as this, is too problematic to implement for such a feature.

The new structure-selection GUI does have a search-box though, which makes selecting a specific structure from a long list much easier when you know the name.

I might revisit this in the future if I can find a clean way to implement it that doesn't actually touch the underlying file-system.