--create-metadata: use yaml.dump() and (if exists) template.yml

To enable maintainers using their specific presets for Metadata, this MR provides 2 commits:

• 732de91a switches to yaml.dump() for metadata file creation, as this way "empty values" are preserved
• 4d375a58 introduces user-defined presets via a template.py file located in the repository root (if this does not exist, built-in presets are used)
• dd694908 fixes the issue that empty built-in presets would always be set (still setting non-empty/non-default attribute "Name" of course, as that's extracted from the .apk)