fix: scroll optimize

[ivanfei-1]

The super_sliver_list package seems to fix this long existing problem mentioned in pr #162 and issue #201. Since the branch is long gone, I am opening a new one here.

I have tested the performance on the material-2 branch, where the improvement is significant.

Before merging this branch, you should test on your own device with a different version of flutter.

[w568w]

I have not got a suitable post to test with. #60678 is deep enough but most floors are simply plain texts. We may need a post with many floors which mostly have complex Markdown content to reproduce parsing performance degradation.

Some problems noticed:

• On Android, in a very deep post (e.g. #60678), if "jump to the bottom" is tapped and finding any floor A which refers to an earlier floor B in the same post, trying jumping to floor B by tapping "Locate" in A's reference will always scroll the list to the top, rather than scroll to B.
  I guess it may have something to do with lazy loading feature of SuperListView, which will not load the previous floors when scrolling to bottom (i.e. their heights are all zero), and thus we mistake the (First floor -> B floor)'s height sum as zero, too.
  Edit: Not the case. The top height seems to be calculated correctly:

  I/flutter (23367): itemTop: 1685.0
  I/flutter (23367): itemTop: 1211.0
  

• Tapping "jump to the bottom" twice will make the list only show the first 10 items.

[w568w]

*sigh*

I think we need to overhaul the entire PagedListView (and StateKey), because there are some implementations that don't follow the Flutter state specification at all, and there are a lot of hacks. They're very likely to cause mysterious bugs.

[w568w]

Update: the 2nd problem seems to be introduced in 2d260ce, where the _hole.floors!.prefetch is reset to only first 10 items during each build.

Anyway, I will rewrite this part. It feels really awkward now.

[ivanfei-1]

LGTM. Basic features have been tested, and no bug is found so far. I think we can merge this branch before thorough testing.