enhancement: let scroll notifications bubble

[harkairt]

Currently the onNotification callback returns true, making ancestor unable to tap into scroll events.

        onNotification: (ScrollUpdateNotification scrollUpdateNotification) {
          if (isAlmostAtTheEndOfTheScroll(scrollUpdateNotification) &&
              isScrollingDownwards(scrollUpdateNotification)) {
            if (!isLoadMoreBeingCalled) {
              performPagination();
            }
          }
          return true;
        },

I have a widget which conditionally elevates the AppBar when the content is scrolled. But it does not work with PaginableListViewBuilder.

~Sorry for not making a PR.~ I made one but could not push branch :D Thanks for the package!

[harkairt]

thanks :)