In 1.8.5 it is now possible to reference directly to enum member.
Hooray! Also added explicit @ref here and there, fixing some referencing
bugs along the way.
Also not through pointer, but throught const&, allows implicit
conversion from Image2D and Trade::ImageData2D, which is good.
Bumped plugin interface version a bit, as this is not so drastic change
in behavior.
Implementation is moved into private virtual `do*()` functions and
public interface does additional sanity checks around them. Exporting to
file is by default done in base implementation, which takes exported
data and then saves them to the file.
Added unit test for file export, bumped plugin interface version to 0.2.