treeview: Make multipress gesture accept all buttons
treeview: Make multipress gesture accept all buttons

That gesture is meant to handle clicks on multiple buttons, so unset
the GDK_BUTTON_PRIMARY default. Also, remove unnecessary boilerplate
with the new GtkGestureSingle/GtkEventController defaults.

https://bugzilla.gnome.org/show_bug.cgi?id=734285

chergert/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,Lyude/gtk-,grubersjoe/adwaita,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,Lyude/gtk-,Adamovskiy/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,alexlarsson/gtk,ahodesuka/gtk,Lyude/gtk-,alexlarsson/gtk,Adamovskiy/gtk,Lyude/gtk-,jessevdk/gtk,ahodesuka/gtk,Adamovskiy/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,ahodesuka/gtk,alexlarsson/gtk,davidgumberg/gtk,Lyude/gtk-,grubersjoe/adwaita,ahodesuka/gtk,jadahl/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,jessevdk/gtk,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,ahodesuka/gtk,ahodesuka/gtk,jessevdk/gtk,Adamovskiy/gtk,jessevdk/gtk,jadahl/gtk,jessevdk/gtk,Adamovskiy/gtk,chergert/gtk,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,jigpu/gtk,davidgumberg/gtk,jessevdk/gtk,Lyude/gtk-,alexlarsson/gtk,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,jadahl/gtk,jessevdk/gtk