Undo redo refactor

[TanLeYang]

Closes #139

Change how undo and redo works. Now all the commands will call the method Model#save() to enable them to be undoable instead of each individual method in Model saving state on its own.

Every command is now undoable other that help and exit, tried it out a bit and it feels more natural that every action made can be undone compared to how it was previously.

Furthermore, undo and redo is now improved to be able to undo back to exactly how the screen was like for the user before he made the command. This is done by mapping each stored State to the correct view type for the Ui to use.

[codecov-io]

Codecov Report

Merging #140 into master will increase coverage by 0.34%.
The diff coverage is 96.82%.

@@             Coverage Diff              @@
##             master     #140      +/-   ##
============================================
+ Coverage     70.97%   71.32%   +0.34%     
- Complexity      734      745      +11     
============================================
  Files           125      127       +2     
  Lines          2384     2420      +36     
  Branches        270      271       +1     
============================================
+ Hits           1692     1726      +34     
- Misses          595      596       +1     
- Partials         97       98       +1     

Impacted Files	Coverage Δ	Complexity Δ
...a/seedu/bookmark/logic/commands/CommandResult.java	85.00% <ø> (-2.50%)	11.00 <0.00> (ø)
src/main/java/seedu/bookmark/model/Model.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...a/seedu/bookmark/model/history/HistoryManager.java	85.71% <ø> (ø)	21.00 <0.00> (ø)
...rc/main/java/seedu/bookmark/logic/ViewManager.java	84.61% <84.61%> (ø)	5.00 <5.00> (?)
src/main/java/seedu/bookmark/logic/ViewType.java	100.00% <100.00%> (ø)	1.00 <1.00> (?)
...java/seedu/bookmark/logic/commands/AddCommand.java	100.00% <100.00%> (ø)	9.00 <0.00> (ø)
.../seedu/bookmark/logic/commands/AddNoteCommand.java	95.00% <100.00%> (+0.26%)	12.00 <0.00> (ø)
...va/seedu/bookmark/logic/commands/ClearCommand.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
...in/java/seedu/bookmark/logic/commands/Command.java	100.00% <100.00%> (ø)	4.00 <3.00> (+3.00)
...a/seedu/bookmark/logic/commands/DeleteCommand.java	100.00% <100.00%> (ø)	7.00 <0.00> (ø)
... and 15 more

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[angrybunny123]

LGTM! Nice job!

[pennhanlee]

LGTM!