add pgup/pgdn keys and fix history in multiline edits
Scroll to correct position when fetching more lines

Fixes #406
Merge pull request #448 from glowing-bear/fix-scrolling-fetchMoreLines

Scroll to correct position when fetching more lines
Merge pull request #445 from ailin-nemui/up-down-pgup-pgdn

add pgup/pgdn keys and fix history in multiline edits
Bufferlist design attempt