Still needs some fixups:
* UI is incomplete
* Investment transactions are unbalanced initially
* OFX imports don't detect if one of the description fields for a
transaction is empty (to fall back on another)
* I'm sure plenty of other issues I haven't discovered yet
* Only update the field that was modified when one is changed instead of
deep-copying the entire Transaction object
* Create a new AmountInput text box to allow for more intuitive editing
of amounts.
