It was a bad idea to bind width/height with pixel density. These are separate
things. You may want to set specific pixel dimensions while keeping a given
resolution.
Moreover I am now properly storing aspect ratio in the widget, otherwise with
integer computation, we are just losing too much precision and the ratio is in
fact changing constantly as you change dimensions.
The code comes from plug-ins/common/file-pdf-load.c and apparently it used to be
in libgimpwidgets (very long ago). I'm copying it to its own file and massively
improve the code (depending on property binding which makes the behavior much
more robust).
Still I left it as private because I don't want to say the API is finale without
having tested it a bit more. But eventually we should make it public for
plug-ins to use it directly too. When this happens, it should get back to
libgimpwidgets.
