This changes the scrolling to be done on the draw, when the height is
updated, ensuring that the selected item is kept on screen during
resizing.
Also, this ensures that messages will fill the screen when resizing the
window, for instance, shrinking and then growing drags down more
messages if possible.
This is a transplant of the dirlist scrolling logic.
Fixes ~sircmpwn/aerc2#205. Many functions do a nil check on the store,
so this changes Store() so it returns nil when msglist is nil.
It also places the Scroll() behind the nil check in the next-message command.
https://todo.sr.ht/~sircmpwn/aerc2/205
