Display schedule options as tree

[yevagorbachev]

Display all of the possible sections of a new course that would fit into a given schedule as children of the given schedule. Easier to go through than many similar-looking schedules.

[mxmeinhold]

This would be pretty neat.

I can mock this better later, but I'm envisioning a UI where schedulemaker picks either the next schedule item with the fewest alternatives (possibly after resolving conflicts) or an arbitrary item, and adds all of its alternatives to a tree. This makes for a relatively intuitive UI (I think), and automatically achieves grouping of similar schedules.

We could even do simplification on the tree, i.e. if the raw tree is like so:

* a
  * b01
    * c
      * d
  * b02
    * c
      * d

it could be collapsed to

* a, c, d
  * b01
  * b02