Draft: app/widgets: Make Dashboard group data widget use .ui xml

First attempt at converting parts of GIMP widgets to declarative definition using GtkBuilder via .ui XML files.

This affects a small part of the Dashboard for tracking system resources used by GIMP: the widget containing an expander, meter with graph and value labels.

This version of the change is not ready for merging as I left out the GimpFrame widget because I suspect GtkBuilder wouldn't like it much. That is a serious shortcoming that I'd like to amend in the future iterations. The main purpose of submitting this MR is to find out if the move to .ui XML files is something the maintainers desire. If yes, I'd like to slowly go through other parts of the code base and continue with the conversion.

Motivation: I find separating the functional part of GIMP from the "only" visual part of it useful as it greatly reduces the complexity of source files.