The expander arrow (triangle) in TreeView

["Roger Morgan" <r morgan gmx ch>]

How does one select an alternative to the default triangle expander arrow in GtkTreeView?

For example, how can I change it to the + with closed folder icon, or - with open folder icon, similar to 
what Firefox uses in the Bookmarks | Organize Bookmarks display? I've checked the docs, but there seems to be 
nothing applicable.

Roger

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