Use a gadget for the button, and for the indicator.
A complication here is that GtkCheckButton (and
GtkRadioButton) have a totally different appearance
depending on the ::draw-indicator property. If an
indicator is not required, we just reuse the
GtkButton gadget.
This mostly works; some minor sizing issues left, e.g. cranking
up the indicator-size causes the checkbutton grid in testgtk
to overlap.