Prepare for GTK4 port
Adapt some code so that they are the same in both GTK3 and GTK4.
The TreeModel changes are already done in !169 (merged) (which is required for GTK4 port)
Adapt some code so that they are the same in both GTK3 and GTK4.
The TreeModel changes are already done in !169 (merged) (which is required for GTK4 port)