Currently, the RadCalculatorPicker's TextBox displays only the currently typed number and the result when you commit the expression. Add a new mode that allows changing this behavior. The mode should allow displaying the currently type operator too. For example, when you type 123 this will be visible in the TextBox. This is the current behavior. Then when you press a sign (like +), this sign should replace the number. If you type another number, the sign should be changed with the number.