textview: Convert gestures' coordinates to text window ones
textview: Convert gestures' coordinates to text window ones

And make some code shared between multipress/drag gesture.

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