core: Change NumPad.qml

Update NumPad.qml to include both the right-side
and left-side column in the same file.

Add a backspace button.
algebra_by activity: Update activity.js

Change the logical condition to retrieve operands.
Add conditions to validate the answer.
algebra_by: Update Activity.qml

Change the QML to accommodate changes made to NumPad.qml
Add condition to reset the operands if the child enters the correct
answer.