Allow raw images to be passed to `#image`

Adds detection of raw data to be loaded into an image view. This allows
users to display images without writing them to disk first.

* Tries to detect binary data by checking encoding
* Continues with loading assuming the passed String is a path otherwise

* Removes the image mock from the test suite and replaces it by a real
image

* Tests for raw loading behaviour
Merge branch 'feature/binary_images' of github.com:skade/shoes4 into skade-feature/binary_images