Rewrite image property page as an extension

The image property page uses the extension API, but is not built as an extension, which would cut down build times and required dependencies when doing minimal builds. Beside that, some changes in the actual extension library were introduced, both technically and organizationally. By “automating” things like type definitions, the codebase is easier to navigate and adding a single-include header makes it less painful to write extensions. The documentation could use more work, but for now some shuffling around and cleaning up should be enough.

While at it, the send-to extension was also cleaned up a little and a directory structure for extensions was created, should there ever be new ones.