It's still basic but will help to share code for support of various vector-able
formats, such as the logic for dimensioning them, but also the generated GUI.

Not only this, but we are paving the way for the link layers (though it'll be
after GIMP 3, we want plug-in procedures' API to stay stable) by giving a way
for a plug-in procedure to advertize a vector format support. This way, the core
will know when a source file is vector and can be directly reloaded at any
target size (right now, in my MR for link layers, the list of "vector" formats
is hardcoded, which is not reliable).