Provide common implementations for two operations that all Python
FileHandlers will typically use.
- `poll_file_object_drop` To be used inside the FileHandler `poll_drop`
callback
- `invoke_popup` To be used inside the Operator `invoke` callback
The above code closely mirrors what is currently done inside the
existing Blender c++ FileHandlers.
Pull Request: https://projects.blender.org/blender/blender/pulls/119774
af7a34dee7