Scroll views up if needed to show all their content

There are many situations where this can arise. Some examples are:
- the terminal window is small, and you are showing a view that shows more
  content than fits into the view port, and the view is scrolled all the way
  down; now you resize the terminal window to a taller size. Previously, the
  scroll position of the view would stay the same, so it would add blank space
  at the bottom; now it will scroll to fill that blank space with content
- expandFocusedSidePanel is on, you go to the bottom of a list view, now switch
  to a different panel, then scroll that (now unfocused) panel all the way down
  with the scroll wheel; now you focus that panel again. It becomes larger
  because of the accordion behavior, but would show blank space at the bottom.

And probably others that I can't remember right now. I only remember that I
always found it confusing to look at a view that had blank space at the bottom
even though it had more content to scroll into view.

pkg/gui/types/context.go

@@ -71,6 +71,9 @@ type IBaseContext interface {
 	// determined independently.
 	HasControlledBounds() bool
 
+	// the total height of the content that the view is currently showing
+	TotalContentHeight() int
+
 	// to what extent the view needs to be rerendered when its width changes
 	NeedsRerenderOnWidthChange() NeedsRerenderOnWidthChangeLevel