app: various GimpDashboard improvements
Refactor GimpDashboard to autogenerate the UI based on a description of the different variables, fields, and groups. Allow individual groups to be expanded/collapsed, and individual fields to be enabled/disabled. Save the relevant state in the dashboard's aux-info. Add fields for the new GeglStats properties, as per GEGL commit 25c39ce6c9bb618f06ac96d118e624be66464d74. The new fields are not enabled by default. Add "reset" action, to clear the history, and reset cumulative data.