Allow defining an invisible resize area for GtkPaned's handle.
Allow defining an invisible resize area for GtkPaned's handle.

This should allow theme developers to use a very small width for
the resize handle, but still let users easily move the handle by
defining a wider resize area.

The additional resize area follows the "margin" style property.

https://bugzilla.gnome.org/show_bug.cgi?id=728073

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