Bug 498010, 546005 - fix assertion in gtk_tree_view_set_cursor_on_cell
Bug 498010, 546005 - fix assertion in gtk_tree_view_set_cursor_on_cell

Instead of failing with warning on !tree_view->priv->tree, return
silently when tree_view->priv->model is NULL.  Clarified in the
documentation that for invalid paths (and every path is invalid when no
model is set), the function will fail silently although the current
cursor will be unset.

Fixes bugs 498010 and 546005.

nacho/gtk-,davidgumberg/gtk,johne53/MB3Gtk-2,Adamovskiy/gtk,chipx86/gtk,jigpu/gtk,jadahl/gtk,Adamovskiy/gtk,grubersjoe/adwaita,davidgumberg/gtk,ebassi/gtk,ebassi/gtk,jessevdk/gtk,msteinert/gtk,alexlarsson/gtk,ebassi/gtk,jessevdk/gtk,bratsche/gtk-,simokivimaki/gtk,bratsche/gtk-,jessevdk/gtk,Distrotech/gtk2,jadahl/gtk,Lyude/gtk-,chergert/gtk,jessevdk/gtk,Adamovskiy/gtk,ahodesuka/gtk,Unity-Technologies/gtk,jigpu/gtk,chipx86/gtk,Adamovskiy/gtk,chergert/gtk,chergert/gtk,Sidnioulz/SandboxGtk,bratsche/gtk-,grubersjoe/adwaita,bratsche/gtk-,jigpu/gtk,jessevdk/gtk,Distrotech/gtk2,Lyude/gtk-,Distrotech/gtk2,Distrotech/gtk,Adamovskiy/gtk,ebassi/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,jigpu/gtk,davidgumberg/gtk,zsx/gtk,davidt/gtk,bratsche/gtk-,ahodesuka/gtk,johne53/MB3Gtk-2,grubersjoe/adwaita,msteinert/gtk,nacho/gtk-,msteinert/gtk,simokivimaki/gtk,Distrotech/gtk,alexlarsson/gtk,simokivimaki/gtk,Lyude/gtk-,Distrotech/gtk2,Adamovskiy/gtk,davidt/gtk,ahodesuka/gtk,Lyude/gtk-,Lyude/gtk-,davidt/gtk,simokivimaki/gtk,chipx86/gtk,nacho/gtk-,Distrotech/gtk,Lyude/gtk-,Unity-Technologies/gtk,ebassi/gtk,Lyude/gtk-,davidgumberg/gtk,alexlarsson/gtk,ahodesuka/gtk,Distrotech/gtk,davidt/gtk,chergert/gtk,davidt/gtk,ahodesuka/gtk,zsx/gtk,bratsche/gtk-,jessevdk/gtk,simokivimaki/gtk,Sidnioulz/SandboxGtk,chergert/gtk,davidgumberg/gtk,grubersjoe/adwaita,ahodesuka/gtk,Unity-Technologies/gtk,alexlarsson/gtk,Lyude/gtk-,jadahl/gtk,zsx/gtk,jigpu/gtk,nacho/gtk-,chergert/gtk,alexlarsson/gtk,Unity-Technologies/gtk,simokivimaki/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,nacho/gtk-,ahodesuka/gtk,Adamovskiy/gtk,zsx/gtk,Adamovskiy/gtk,grubersjoe/adwaita,grubersjoe/adwaita,ahodesuka/gtk,msteinert/gtk,grubersjoe/adwaita,davidt/gtk,johne53/MB3Gtk-2,davidgumberg/gtk,Distrotech/gtk2,alexlarsson/gtk,ebassi/gtk,jadahl/gtk,jadahl/gtk,davidgumberg/gtk,msteinert/gtk,zsx/gtk,Distrotech/gtk,chipx86/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,chipx86/gtk,jadahl/gtk,jessevdk/gtk,msteinert/gtk,grubersjoe/adwaita,chergert/gtk,Distrotech/gtk2,jigpu/gtk,chergert/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,jadahl/gtk,jigpu/gtk,johne53/MB3Gtk-2,Sidnioulz/SandboxGtk