Implement project Essential_L12.2 (Events - WPF - Calculator)

@kostiantyn-kuzin please review
This part of the code should remain here, in the click handler. Create class OperationRequest with 3 members: Operand1, Operand2 and Operation. Operation should be an enumeration with the following content: Add, Subtract, Divide, Multiply. Operand1 and Operand2 should be double.

In Button_Click you should correctly parse the user input and create a new OperationRequest.