GladeCommand: Experimental - properly record child type for removal commands.

This seems to be working well but might cause problems (let's call it
a red flag if things go wrong and we need to bisect), it fixes drag & drop
undo/redo for GtkExpander (Drag and drop the expander label into the
expander content and undo/redo/undo/redo, this was broken, and this commit
fixes that and also improves things for the currently experimental headerbar
support).