Give edit systems access to the level index

[idanarye]

Most straightforward way - make it a resource.

The idea is to allow levels to refer other levels. For example - a door entity can have a field with the name of another level that door leads to.

[idanarye]

I'm not sure I want the actual level index itself exposed (since I want to eventually remove it in favor of #34), but it'd probably be a good idea to expose a BTreeMap of the level files paths.

• Why tree and not hash? Because they will be displayed in dropdown combo boxes, so I don't want a changing order - and a lexical order would actually be useful. I also don't want users to rely on the order from the level index.
• Why map and not set? So that later on I can put data in the values, for edit systems to rely on.

[idanarye]

Actually, I'll just start with a BTreeSet but keep it encapsulated for now, so that the only interface will be YoleckEditableLevels::paths() (and YoleckEditableLevels::contains())