When composing a new thread and writing more than one line, the chat
wouldn't scroll to make the editor cursor visible again. My original
guess for when the scrolling code should run was a bit too restrictive.
When pressing ctrl+y while cursor = Cursor::Bottom, the screen would
not scroll. Instead, the cursor would only jump to the lowest message on
the screen. This was caused because the blocks were re-layouted after
scrolling, starting from the Cursor::Bottom, thus resetting the scroll
to the bottom of the screen.
When sitting at the bottom of a room with cursor = Cursor::Bottom, a
rerender would make the cursor jump to the lowest visible message. This
of course should only happen when the screen is scrolled, not on almost
every rerender.
This also moves the Cursor definition back to the cursor module, and
modifies it to include info about the last non-editor/non-pseudo
position in editor/pseudo cursors (to be used when editing or waiting
for the server reply is aborted via Escape)
