There is currently no property widget which just creates a
GimpSpinButton with a label. Just as the GimpScaleEntry was needed, this
one is as well.
I am creating a GimpLabeled abstract class which will represent various
widgets with a label. While this may seem a bit over-engineered for such
a basic feature, this will actually bring some consistency and a common
parent. In particular this can be used to get the GtkLabel with a common
interface to add them all in a common GtkSizeGroup when generating
dialogs automatically, hence make dialogs with properly aligned labels
and edition widgets.
