ShumateMarker: Add a property for the child widget
At the moment it looks like gtk_widget_insert_before/after is the only way to add a child to a marker. However, the docs say
This API is primarily meant for widget implementations; if you are just using a widget, you must use its own API for adding children.
So it's probably better to have a
ShumateMarker:child property like other one-child GTK widgets.