core: NumPad.qml: Replace "backspace" with "<-"

Use HTML hex code to replace the word "backspace" with the arrow.
algebra_by: Update activity.js

Remove the currentSubLevel() methos since it is not used anywhere.
algebra_by: Update Activity.qml

Fix operands refresh when level number is increased.
Try to add the "Iam ready" button feature

TODO:
Balloon time expired is not detected yet.
Fix "Iam ready" button.
Review code.