URLImageDescriptor and FileImageDescriptor

This fixes #679: It restores the Image#equals contract, which depends on
non-equal providers per image.

This also introduces a private getImageData overload that is reused.

The corresponding tests are modified, too: Now they assert non-sameness
of the provider instead of sameness. New tests are provided that assert
that images that are created from a given ImageDescriptor are not equal.

As a drive-by fix, this also adds UrlImageDescriptorTest to
AllImagesTests.

To preserve the testability of their underlying providers, the two
affected ImageDescriptor classes return them now through their
getAdapter methods (And so so similarly by creating fresh instances).

Signed-off-by: Daniel Krügler <daniel.kruegler@gmail.com>