Global SDL

This changeset implements GlobalSDL. Note that GlobalSDL should stored separately from both the AgeSDLHook PythonFileMod state and the local VaultSDL node. Therefore, I have implemented it as a late-merge operation. Please test that I did not accidentally break the crazy merging that @branan already implemented.

How to use this:
- A new `globalsdl` command has been introduced with the following arguments: `agefilename`, `variable`, `value`.
- Omitting the `value` argument will cause the value to be reset to its default.

This closes #22.