Automatically generate IDs and UIDs for entities and properties

[nalenz-objectbox]

See #18.

[vaind]

While dart can be run as a script, it's supposed to be compiled for "dartaotruntime" for production. Thus it can't really just read the model.json from the current directory. Or am I missing something?

All the other bindings handle this by including all necessary information (ID, UID, Type in DB) in the generated files.

However, it seems the model file could be embedded in a package with - https://pub.dev/packages/resource.

Since that still comes with a performance & size drawback, consider whether using parts of the original generator code would make sense, thus completely avoiding having to deal with resources and possibly two ways of loading the file (compiled vs in VM)

[vaind]

Also don't forget to rebase/merge changes from dev

[vaind]

The remaining things before this PR can be merged:

• rebase/merge the dev branch
• tests - e.g. generate code for multiple entities, not just one. Swift & Go bindings do tests by comparing the generated code & model to an "expected" version for each test case (a folder here: https://github.com/objectbox/objectbox-go/tree/master/test/generator/testdata). No need to go to such extent but at least some tests should be there
• at least some basic docs for the new classes

[Buggaboo]

I'm currently working on doing Queries, is the boolean / byte / double / long implementation for generating properties included in this branch?

[vaind]

Merging this into dev, please add the missing class docs directly on dev branch @nalenz-objectbox