As expected, it is made to reuse shared code for every GimpVectorLoadProcedure.
In particular, they all need to choose dimensions to load at, so we are sharing
a same GimpResolutionEntry widget logic everywhere now.

I am in fact still very unsure about the code logic for this widget by the way
for these reasons:

* It still puts too much emphasis on the "resolution" (pixel density) part,
  which makes people believe it's important, while they should in fact choose
  the pixel dimensions most of the time and not care about the pixel density.
* Right now we can't break ratio (which in fact was already impossible in most
  vector format plug-ins we had). Do we want to add a chain and allow this?
* If we consider the pixel density as the one we want to set the document with
  (which may not be the same thing as the one from when we load the document),
  we also want to break link between width/height dimensions and pixel density.
  Right now we can't (updating one field updates the others too).
* There is always this issue of precision with pixel density vs. pixel
  dimensions because we don't necessarily find the same values when computing
  from one side to another because of lack of precision and this confuses
  people.
* Finally there is the question of multi-page documents (e.g. PDF) where the
  chosen dimensions are the document dimensions whereas each page may have a
  different size which has to be recomputed independently and this got me
  off-by-one errors. I think I'll need to review a bit the logic, but I'll do
  once I've ported all the vector format load plug-ins first to see the most
  common usages.