API fixes to work with plain dicts read by any YAML parser

Since metadata files are now YAML, programs should be able to just read in the .yml files, then use them with fdroidserver methods without having to know about the App and Build classes or other internal oddies. The App and Build classes just provide syntactic sugar by allowing dict keys to be accessed as attributes.

The .txt format was the last and only place where the lowercase builds was used, this converts references everywhere to be Builds, matching the metadata files. This makes it possible to load metadata YAML files with any YAML parser, then have it possible to use fdroidserver methods on that data, like metadata.write_metadata().

The test files in tests/metadata/dump/*.yaml were manually edited by cutting the builds: block and putting it the sort order for Builds: so the contents should be unchanged.

WIP because it is based on !837 (merged)