Fix Update UI Bug

[RobotWizzard]

Adds a field in Model to keep track of the last viewed client. This field is updated when commands involving rental information are executed.

Commands updated:

• rview
• radd
• rdelete
• cdelete
• clear
• redit
• cedit

Fixes #127

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...du/address/logic/commands/DeleteClientCommand.java	0.00%	2 Missing and 1 partial :warning:
...eedu/address/logic/commands/EditClientCommand.java	0.00%	1 Missing and 1 partial :warning:
...eedu/address/logic/commands/ViewRentalCommand.java	0.00%	1 Missing :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...seedu/address/logic/commands/AddRentalCommand.java	93.75% <100.00%> (+0.20%)	8.00 <0.00> (ø)
...ava/seedu/address/logic/commands/ClearCommand.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
...du/address/logic/commands/DeleteRentalCommand.java	100.00% <100.00%> (ø)	11.00 <0.00> (ø)
...eedu/address/logic/commands/EditRentalCommand.java	93.96% <100.00%> (+0.05%)	11.00 <0.00> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	94.91% <100.00%> (+0.36%)	27.00 <3.00> (+2.00)
...eedu/address/logic/commands/ViewRentalCommand.java	21.73% <0.00%> (-0.99%)	2.00 <0.00> (ø)
...eedu/address/logic/commands/EditClientCommand.java	87.35% <0.00%> (-2.06%)	14.00 <0.00> (ø)
...du/address/logic/commands/DeleteClientCommand.java	86.36% <0.00%> (-13.64%)	7.00 <0.00> (ø)