Basic Query (Builder) support

[vaind]

Goal: minimal solution including a query builder and a reusable query object. For reference, please have a look at any of ObjectBox Go/Dart/Swift/Java API and implementations.

Functionality:

• QueryBuilder to allow "equal" criteria for string and int properties (pass model property)
• QueryBuilder allows build()-ing a Query object
• Query object has methods: find(), count(), remove().
• Optional: Query has offset() and limit() methods - just calling the C-API which handles all the details

Steps to accomplish:

• add the required C-API function definitions to c.py, based on objectbox.h - have a look at objectbox-go or objectbox-dart to see the needed c-api functions, e.g. obx_query_builder(), obx_qb_equals_string(), obx_query(), obx_query_find()
• create the python query builder and query classes to the objectbox directory
• add simple tests to the tests directory, to actualy check the new code works

[greenrobot]

Done in 44721d9