Revision 1703267

PIVOT-972:  Fix some more bugs with the calculator tutorial:
* Implement the difference between "Clear" and "All Clear".
* Highlight the current operator button (by changing border color).
* When two operators are pressed in a row, just change operators,
  don't do the prior operation.
* Fix backspace not to do it twice, and to work properly when clearing
  the last digit.

This is a merge of revision 1688484 from trunk to branches/2.0.x.