AbstractionList outline expand features

[bryanph]

This depends on #162

Features:

• Enter => create a node below

1. in middle: split and save both
2. at beginning: create an empty node above (don't sync)
3. at end: create an empty node below (don't sync)

• A + button when there are no child nodes to add the first node
• Pressing tab => move below hierarchy
• Pressing shift-tab => move up a hierarchy