Bugfixes, Obj-C support lib prefix and Obj-C public/private separation

Fixed bugs:
- The Obj-C generator did not apply any include prefixes
- The Obj-C generator did not apply C++ namespaces correctly:
  - References to the C++ base classes were without namespaces
  - The `+o xxObjcProxy` classes were not wrapped in nested namespaces correctly
  - The C++ return types of `+o xxObjcProxy` had no namespaces applied to them
- The JNI generator did not apply `--jni-base-lib-include-prefix` to `djinni_support.hpp`
- Changed the default namespace for Obj-C code generation from `dropboxsync` to `djinni_generated` to match the JNI option

New Features:
- Added `--objc-base-lib-include-prefix` similar to `--jni-base-lib-include-prefix` for the Obj-C support library
- Added `--objc-private-out` and `--objc-include-private-prefix` to allow for separation of public and private Obj-C output