ProgressHandler support in C API

Assimp exports the `ProgressHandler` class and offers `Importer::SetProgressHandler()` to set a custom progress handler.

This, however, requires a C++ class with virtual functions and there is no way (to my knowledge) to use this functionality from the C interface in `cimport.h`.

I.e. from C, it is impossible to
1. display progress information
2. cancel imports.

It would be handy to have C support for progress: Let Assimp internally derive `ProgressHandler` to store a pair of (C function pointer + user context) similar to `struct aiFileIO`, then forward all updates to the C function pointer. I don’t know how that pair would be passed to `aiImportFileFromMemoryWithProperties()` without breaking the API, though.

Start with cimport.h and the ProgressHandler/Importer::SetProgressHandler() entry points, then inspect aiImportFileFromMemoryWithProperties() and the existing aiFileIO callback pattern. Define how a C callback and user context would be registered without breaking the API; done means C callers can receive progress updates and cancel imports.

Written by the indexing model from the issue text.