Bookmarks don't show on real beatmaps, but they do show in test scenes
(namely `TestSceneEditorSummaryTimeline`).
Also does some more changes to adjust the markers to the latest updates
to other markers.
- Use slightly different hue of green to distinguish from difficulty
control points. The colour is still not ideal, but picking
a distinctive enough hue is pretty hard.
- Place the preview time part at the bottom rather at the top.
Not sure why it was at the top; not only could it overlap with the
control points, but it also looked quite badly misaligned there
when bookmarks were displayed at the bottom.