Long widget scroll in ScrollablePane

Currently ScrollablePane is seemingly only designed for the widget-unit scrolling, hence if widget doesn't fit in screen we're screwed.

In that case as soon as it goes beyond screen autoscrolling stops and no matter what we do on vertical_scroll it does nothing.

I wonder if there's way to wrap ScrollablePane in BufferControl so I can scroll it line by line, or just adding line-by-line scroll in ScrollablePane is solution. In that context could be related to #1861

So far half-broken workaround seems to be:

• Using dummy widget at bottom of long widget so scroll follows it as widget grows, but then still can't scroll up line by line.

• Using widget's .control.move_cursor_down() - scroll still stops

• Using widget's .buffer.cursor_down() - scroll still stops

Start by locating ScrollablePane and tracing how vertical_scroll behaves when a widget exceeds the screen. Compare the possible BufferControl approach with line-by-line scrolling and review related issue #1861. Done means a long widget can be scrolled vertically line by line beyond the visible screen, including upward scrolling.