Create groups of tasks (a.k.a. sub-items)

[pwizla]

The goal is to get this kind of list:

• Bake a cake for my friend's birthday
  • Look up some recipes over the Internet
  • Check what ingredients I already have at home
  • Go to the store and buy the missing ingredients
  • Actually bake the cake!
• Renew my gym membership

No more than two levels 2:

• Level 1 task
  • Level 2 task
  • Another Level 2 task
• Another Level 1 task

Requires some parent-child model.
Should support drag'n'drop: dropping over an existing task makes it a sub-task.
(and maybe keyboard shortcuts later: Tab = indent, Shit Tab = unindent).