Generator rework

[vaind]

This PR is based on #74, please merge it first and then change the target of this PR to dev branch

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Fixes #55

See issue for more context, in short: the generator wasn't usable when entities were spread across multiple files.

Changes the code generator to two steps:

1. EntityResolver analyzes entities, producing .objectbox.info files in cache (not in project sources).
2. CodeBuilder collects those, merges them into objectbox-model.json and creates a single objectbox.dart output file that should be included by the user code.

This PR also:

• fixes removal of properties,
• adds removal of entities,
• some internal changes necessary to accept the new model class.

TODO:

• update examples
• fix/change generator tests
• update doc/*.md and move to appropraite locations as those are code related, not user docs

[vaind]

P. S. I did use git mv but it still got confused so some generator sources are not diffed properly. Sorry about that.