vim: x should never cause non-current lines to change.

This will no longer start deleting the next line or the previous line.
It will work forward or backwards on the current line only.
vim: implement basic "o" and "<Shift>o" support.

This still needs to interacti with the auto-indent provider so that
we can get the right character offset.

That will probably take some changes to the auto-indent layer so that
we can avoid synthesizing GdkEventKey events.