Savefile layout for folders

For Lv2, we need to be able to load whole directories, and they also need to be synchronizable into XML. This is (almost) completely independent from #4922 .

# Sketch

Basic idea:

```

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There would be only 3 possible types right now:
* utf-8 (the encoding Lv2 specifies)
* binary (anything where encoding is unclear)
* external (if a file shall be outside of the XML, currently unsupported)

# Implementation

Probably `DataFile` is the class to use. Currently, I think that's a class to contain objects ("files") in XML (or compressed XML) format that can be
* loaded from files
* loaded/written into LMMS savefiles

Extending the definition of "objects" to not only contain files, but also directories, might make sense. Loading from directories would mean to simply smash all files from the folder into single `CDATA` sections, like above.

Any opinions or issues you see?

Start by reading the DataFile class and the existing savefile/XML loading and writing paths. Compare the proposed folder representation and its utf-8, binary, and external file types with the Lv2 directory-loading and synchronization requirements, while checking the relationship to #4922. Done requires an agreed design before implementation can begin.

Written by the indexing model from the issue text.