Fix expense and income lists to update automatically

[wltan]

Fixes #128

The root cause of #128 is that a new ObservableList is constructed every time getFilteredExpenseList or getFilteredIncomeList is called. This makes it so that the UI must be responsible for calling these methods to update itself. However, this goes against the philosophy of an ObservableList in the first place.

Instead, inside ModelManager we will maintain two ObservableLists of the correct type parameter (Expense and Income). Any call to getFilteredExpenseList or getFilteredIncomeList will return a wrapper around the same ObservableList every time, instead of constructing a new list.

When changes are made to the filtered expense or income lists, the corresponding internal ObservableLists will be refreshed with the latest values. This unfortunately needs to be done 'manually' within the ModelManager, but at least it is contained within the Model component and is not dependent on any other components.

Pitest: https://ay2021s1-cs2103t-w16-3.github.io/reports/pitest/202010241722/

[codecov-io]

Codecov Report

Merging #168 into master will decrease coverage by 0.01%.
The diff coverage is 87.50%.

@@             Coverage Diff              @@
##             master     #168      +/-   ##
============================================
- Coverage     74.76%   74.75%   -0.02%     
- Complexity      701      702       +1     
============================================
  Files           124      124              
  Lines          2128     2139      +11     
  Branches        226      227       +1     
============================================
+ Hits           1591     1599       +8     
  Misses          448      448              
- Partials         89       92       +3     

Impacted Files	Coverage Δ	Complexity Δ
...021s1_cs2103_w16_3/finesse/model/ModelManager.java	89.69% <87.50%> (-2.17%)	43.00 <5.00> (+1.00)	⬇️

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[ianyong]

LGTM!

[zhaojj2209]

LGTM!