Fix VendorCommand and Menu

[ernestlim8]

Fix bugs with UI not displaying message when command is executed successfully.
Fix Command tests after refactoring.
Add new feature where the current order is cleared when the vendor command changes to a new vendor.

[codecov-io]

Codecov Report

Merging #86 into master will decrease coverage by 0.16%.
The diff coverage is 58.82%.

@@             Coverage Diff              @@
##             master      #86      +/-   ##
============================================
- Coverage     59.35%   59.19%   -0.17%     
- Complexity      517      518       +1     
============================================
  Files           101      101              
  Lines          1961     1968       +7     
  Branches        203      205       +2     
============================================
+ Hits           1164     1165       +1     
- Misses          728      733       +5     
- Partials         69       70       +1     

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/seedu/address/logic/LogicManager.java	61.11% <0.00%> (-3.89%)	3.00 <0.00> (ø)
src/main/java/seedu/address/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...va/seedu/address/logic/commands/CommandResult.java	78.94% <80.00%> (-9.29%)	10.00 <3.00> (ø)
...java/seedu/address/logic/commands/ExitCommand.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...java/seedu/address/logic/commands/HelpCommand.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...va/seedu/address/logic/commands/VendorCommand.java	100.00% <100.00%> (ø)	8.00 <0.00> (+1.00)

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[MorningLit]

beautiful