In GTK, a common scheme is to let a function creating a specific widget
to return a `GtkWidget *`, rather than the specific subtype, since you
often need to call API of GtkWidget, avoiding some useless casts.
For bindings however (and especially bindings to compiled languages),
this is a bit annoying, as you have to explicitly change the type of the
return value (downcast), which is not trivial (or at least desirable) in
each language.
Luckily, we can use `(type ...)` annotation for this use case, leaving
the C API unchanged, while improving the experience for bindings.
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.
