notebook: up/down arrow keys move to begin/end of line at top/bottom of cell...
notebook: up/down arrow keys move to begin/end of line at top/bottom of cell
The behavior is the following:
* If you press UP while on the first row, you go to the beginning of the line.
* If you press UP again (or were already at the beginning of the line), you
go to the previous cell.
* If you press DOWN while on the last row, you go to the end of the line.
* If you press DOWN again (or were already at the end of the line), you go
to the next cell.
This applies to `CodeCell`s and `RawCell`s.
