We really want margins around the scrollable content, not around
the viewport. Make it so by using textview-specific properties.
This is unfortunately a little complicated for top/bottom.
Two changes that sneaked in during the GtkApplication port
made it so that the window would not let you shrink it again
after you've made it larger. This also yielded very surprising
results when unmaximizing the window: it would come back to
have a minimum width slightly larger than the screen, making
maximization fail from then on.
