#10 Add Undo (Ctrl+Z) option in Examination designer

[Orkin]

There's no ability undo the changes in Examination designer.
Add 'Undo' (Ctrl+Z) option in right click menu.

[Volodya Shtenyovych]

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Implemented,
please for problems you found with it create new issues.

[Orkin]

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rev. 651 - Undo/Redo Button is added, but please, implement the keystrokes support:
Undo - Ctrl+Z
Redo - Alt+Shift+Backspace