Fix clear undo

[koh-jx]

Fixes #109.
Due to setAddressBook(), which calls AddressBook#resetData() and resets the PersonList rather than the AddressBook itself.

Also fixed a bug where clear does not update the presence of contacts in tasks.

[codecov-commenter]

Codecov Report

Merging #157 (99bcbf1) into master (83edc0c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #157      +/-   ##
============================================
+ Coverage     58.68%   58.70%   +0.01%     
  Complexity      602      602              
============================================
  Files           121      121              
  Lines          2561     2562       +1     
  Branches        251      251              
============================================
+ Hits           1503     1504       +1     
  Misses          966      966              
  Partials         92       92              

Impacted Files	Coverage Δ
...ava/seedu/address/logic/commands/ClearCommand.java	54.54% <100.00%> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	82.02% <100.00%> (+0.10%)	⬆️

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[jeffsieu]

LGTM