gtk: change GtkIconView to "support" actual trees
gtk: change GtkIconView to "support" actual trees

Remove the check for GTK_TREE_MODEL_LIST_ONLY and make the view
display only the first level of the tree, ignoring all insert, remove
etc. events on the model's branches. This enables generic application
code which always produces GtkTreeStores (also for flat lists) to
display grids of these flat trees.

grubersjoe/adwaita,simokivimaki/gtk,davidgumberg/gtk,ebassi/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,davidgumberg/gtk,ebassi/gtk,jigpu/gtk,simokivimaki/gtk,jigpu/gtk,jadahl/gtk,ahodesuka/gtk,ebassi/gtk,chergert/gtk,ahodesuka/gtk,simokivimaki/gtk,alexlarsson/gtk,msteinert/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,Distrotech/gtk2,msteinert/gtk,ebassi/gtk,ahodesuka/gtk,davidt/gtk,chergert/gtk,bratsche/gtk-,Lyude/gtk-,jessevdk/gtk,bratsche/gtk-,jadahl/gtk,Distrotech/gtk2,Lyude/gtk-,alexlarsson/gtk,alexlarsson/gtk,msteinert/gtk,jigpu/gtk,ahodesuka/gtk,jigpu/gtk,jessevdk/gtk,Lyude/gtk-,msteinert/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,bratsche/gtk-,jessevdk/gtk,davidt/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,davidt/gtk,chergert/gtk,davidt/gtk,simokivimaki/gtk,bratsche/gtk-,chergert/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Adamovskiy/gtk,ebassi/gtk,davidgumberg/gtk,Lyude/gtk-,jessevdk/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,ahodesuka/gtk,davidgumberg/gtk,Distrotech/gtk2,ebassi/gtk,jigpu/gtk,davidgumberg/gtk,ahodesuka/gtk,alexlarsson/gtk,Adamovskiy/gtk,bratsche/gtk-,Lyude/gtk-,grubersjoe/adwaita,simokivimaki/gtk,Distrotech/gtk2,msteinert/gtk,Adamovskiy/gtk,chergert/gtk,davidgumberg/gtk,simokivimaki/gtk,chergert/gtk,jadahl/gtk,msteinert/gtk,Adamovskiy/gtk,Distrotech/gtk2,jigpu/gtk,Sidnioulz/SandboxGtk,Sidnioulz/SandboxGtk,jadahl/gtk,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,jessevdk/gtk,chergert/gtk,Lyude/gtk-,jadahl/gtk,chergert/gtk,davidgumberg/gtk,ahodesuka/gtk,jigpu/gtk,bratsche/gtk-,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,jessevdk/gtk,davidt/gtk,Distrotech/gtk2,Lyude/gtk-,grubersjoe/adwaita,davidt/gtk,davidgumberg/gtk,ahodesuka/gtk