Add a toolbar with some basic actions

We change `scrollTop` to scroll cells into view, but cell's position
is based on template height, which is not correct. Markdown cells
are dynamic, so after we scroll cells into view, they will change
their real height, thus we need to set `scrollTop` again to adjust
the position again.

For code cell, they will call `updateElementHeight` to change its
size. It should pass in `focusedElement` as the anchor element,
then the focused element will keep its visual position.

Add some notebook commands

This reverts commit 540fbabf.