Add confirmation for deleting and clearing all contacts

[Bonzzz3]

Ready for review Note: Unable to increase coverage due to ui constraint (use of alert dialog) Fixes #76

Description

Add confirmation extension for delete and clear command

Key Changes

• [x] Add confirmation window for both delete and clear command and proceed only when user select ok
• [x] Updated and add new test cases for the new addition
• [x] Update user guide with new extension

Future Plan

Change Ui of confirmation window to match with the main window

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.19355% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...main/java/seedu/address/ui/ConfirmationWindow.java	50.00%	7 Missing :warning:
...va/seedu/address/logic/commands/DeleteCommand.java	84.84%	4 Missing and 1 partial :warning:
...ava/seedu/address/logic/commands/ClearCommand.java	71.42%	3 Missing and 1 partial :warning:

Files with missing lines	Coverage Δ	Complexity Δ
src/main/java/seedu/address/ui/UiPart.java	84.61% <100.00%> (+0.61%)	4.00 <0.00> (ø)
...ava/seedu/address/logic/commands/ClearCommand.java	73.33% <71.42%> (-26.67%)	5.00 <4.00> (+3.00)	:arrow_down:
...va/seedu/address/logic/commands/DeleteCommand.java	90.74% <84.84%> (-9.26%)	23.00 <9.00> (+7.00)	:arrow_down:
...main/java/seedu/address/ui/ConfirmationWindow.java	50.00% <50.00%> (ø)	4.00 <4.00> (?)