groundwork and examples for simplification

OperatorSpec, Operator, and OperatorExpression now have simplify()
methods, which use the same paradigm as the toLatex() methods. Example
simplify() methods have been implemented for +, -, *, /, and ^.
removed deprecated operators

Removed the deprecated operators angle, segment, line, ray, and circle,
which were not used by SketchCanvas.
added simplify button to MainWindow
bugfixes for commits b436b8d and fd3ce2e