GtkTextView: various scroll fixes
GtkTextView: various scroll fixes

Due to the introduction of view's margin and padding,
some bugs in scrolling behaviour have come.
This commit fix them.

https://bugzilla.gnome.org/show_bug.cgi?id=753815

jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,chergert/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,davidgumberg/gtk,jadahl/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk