Add parent node behavior

[rzach]

Ctrl-Enter is supposed to add a parent node, but what it does is add a sibling to the existing parent nodes. Since adding another premise (ie adding a sibling to a parent node) is easily accomplished by selecting the parent and the pressing Enter, it may be more useful to make Ctrl-Enter actually produce a parent node which becomes the child of all existing parents, i.e.,

P1   P2
----------
Q

becomes

P1   P2
----------
R
---
Q