ui: Maintain the focus position when zooming DwlTimeline
ui: Maintain the focus position when zooming DwlTimeline

This means the view will remain scrolled at the cursor while zooming,
which is much more natural than jumping scroll position all over the
place.

This is a hacky first implementation; ideally the implementation would
be cleaned up by implementing GtkScrollable directly on DwlTimeline and
dropping our implicit usage of GtkViewport.

pwithnall/dunfell,pwithnall/dunfell