SanderMertens#124 add C++ entity documentation

SanderMertens#124 fix cmake build, add example

SanderMertens#124 add ecs_set_min_count to ecs_vector_t, extend migra…

…tion guide

SanderMertens#124 add target_include_directories to cmake file

SanderMertens#124 add target_include_directories to cmake file

SanderMertens#124 fix C examples

SanderMertens#124 don't prefix entity constants for builtin tags

remove cpptest until tests can be compiled in c++

SanderMertens#78 fix assert occurring with sigs only containing OWNED…

… or SHARED

SanderMertens#37 move bake-specific code to os_api