Implement Purge undo, cleanup

[koh-jx]

undo Purge

• Restores all tasks in their correct order there were in, as well as set filters back to previously selected filters.

Noted bugs

Tasks with the same name were not added due to the equality of tasks affecting indexOf(). This is fixed with Jeff's merged PR bug fix.

Refactor Purge

As previously mentioned in the purge PR, the Model#deleteFilteredTasks method has been refactored to not take in an arbitrary List<Task> and assume that it is the Filtered task list. This is because PurgeTaskCommand no longer provides its own list but just calls this method.

Cleanup undo

• Removed now unnecessary code in various commands.
• Add that filter does not count as a command that can be undone in the undo UG,

[codecov-commenter]

Codecov Report

Merging #90 (907fff1) into master (e753d27) will decrease coverage by 0.37%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master      #90      +/-   ##
============================================
- Coverage     61.98%   61.60%   -0.38%     
  Complexity      535      535              
============================================
  Files           107      107              
  Lines          2057     2068      +11     
  Branches        198      201       +3     
============================================
- Hits           1275     1274       -1     
- Misses          713      725      +12     
  Partials         69       69              

Impacted Files	Coverage Δ
...ava/seedu/address/logic/commands/ClearCommand.java	54.54% <0.00%> (+8.39%)	⬆️
...java/seedu/address/logic/commands/FindCommand.java	73.33% <ø> (+2.74%)	⬆️
...address/logic/commands/task/DeleteTaskCommand.java	63.15% <ø> (+3.15%)	⬆️
...u/address/logic/commands/task/DoneTaskCommand.java	80.95% <ø> (+3.67%)	⬆️
...u/address/logic/commands/task/EditTaskCommand.java	79.74% <ø> (+0.99%)	⬆️
.../address/logic/commands/task/PurgeTaskCommand.java	0.00% <0.00%> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	84.29% <0.00%> (-2.15%)	⬇️
src/main/java/seedu/address/model/TaskList.java	72.00% <0.00%> (-3.00%)	⬇️

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[wlren]

LGTM