Better control for scrolled area size
This topic has been problematic for far too long, and GtkScrolledWindow and GtkViewport have a confusing array of properties yet still fail to do a convincing job.
- Add gtk_scrollable_measure_viewport(), with a vfunc in the scrollable interface
- Make GtkScrolledWindow call that instead of consulting its own properties
- Give common scrolled children properties to influence it, like GtkTextView::min-rows