Add support for multiple configurations

[Peechey]

Fixes #2794 Fixes #899

Description of the problem being solved:

Add ConfigSets, similar to skill, item, or tree sets. This includes a change to the Config xml to use the ConfigSet elems with Input/Placeholder children vs a dump of one set of Input and Placeholder elems. Backwards compatibility accounted for. There's probably some refactoring/cleanup I can do, but I wanted it out there for feedback.

Steps taken to verify a working solution:

• Test new build can create, delete, copy, rename
• Test new build saves and loads and loads correct activeSet
• Test older builds that do not have the new xml structure and validate save successfully
• Test importing and exporting old/new structure builds
• Test custom mods carry over

Link to a build that showcases this PR:

Here's an older build with multiple config changes including Custom Mods

https://pobb.in/HmQ8HnyI9fhT

After screenshot:

[ConKou]

Thought this was coming in 2.39 :(

[ConKou]

Hello, you think this will be ready in time for new league planning?