Create ProjectedBalance to hold ending balances
Add sorted diffs list to track most under/over estimates
Add projected balance calculator and unit tests
Add guards to ProjectedBalance.hpp
Integrate calculation results into application.

In the process, the TransactionAssigner, ProjectedBalance,
and Balance classes needed to have their emitted signals modified
so that the end display models can listen for changes to the base
models.
Merge branch 'task/balance' into qt-rewrite