libgimp: re-promote GimpResolutionEntry in its own file and improve i…

…ts API.

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.