Increase Assets API performance

[bartfeenstra]

• Benchmark everything!
• Consider renaming the file system API to the assets API
• Consider removing functionality not needed for assets management (what exactly?)
• Consider scanning for all files upon first use and then caching the resolved results, e.g. removing overridden files from the resulting file list.
• FileSystem.open() should only take a single path for consistency (also Betty never uses the method itself)
• Make AssetRepository an interface and refactor the existing class into a concrete implementation.
• add a decorator for Project.assets

Blocked by

• Replace FileSystem with AssetRepository #1613