Implement basic GUI for sales tracking section

[ureshiiYing]

Assertions and Logging has also been added at appropriate places.

[codecov-io]

Codecov Report

Merging #103 into master will decrease coverage by 0.70%.
The diff coverage is 21.27%.

@@             Coverage Diff              @@
##             master     #103      +/-   ##
============================================
- Coverage     65.03%   64.33%   -0.71%     
- Complexity      613      617       +4     
============================================
  Files           110      111       +1     
  Lines          2105     2142      +37     
  Branches        246      258      +12     
============================================
+ Hits           1369     1378       +9     
- Misses          649      667      +18     
- Partials         87       97      +10     

Impacted Files	Coverage Δ	Complexity Δ
...address/logic/parser/SalesUpdateCommandParser.java	88.88% <0.00%> (-11.12%)	5.00 <0.00> (ø)
...ava/seedu/address/model/UniqueSalesRecordList.java	87.71% <0.00%> (-5.01%)	27.00 <0.00> (ø)
src/main/java/seedu/address/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...in/java/seedu/address/ui/SalesRecordEntryCard.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...in/java/seedu/address/ui/SalesRecordListPanel.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
src/main/java/seedu/address/model/SalesBook.java	78.72% <42.85%> (-8.46%)	19.00 <1.00> (+1.00)	⬇️
...rc/main/java/seedu/address/model/ModelManager.java	73.11% <50.00%> (+0.29%)	34.00 <1.00> (+1.00)
...rc/main/java/seedu/address/logic/LogicManager.java	57.14% <100.00%> (+1.58%)	4.00 <1.00> (+1.00)
...seedu/address/logic/commands/SalesListCommand.java	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...edu/address/logic/commands/SalesUpdateCommand.java	100.00% <100.00%> (ø)	6.00 <0.00> (ø)
... and 5 more

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[yanbingtao]

LGTM

[WM71811]

LGTM