Add boolean expression SelectionAccessor

So far, SelectionAccessor was only able to handle simple AND expressions like a,as_char&&b,as_char. This is implemented by just splitting the components and maintaining a list. With this MR there is another class SelectionAccessorExpr, that will be invoked in case an expression given to makeSelectionAccessor is more complex than that. It will parse the expression and build a tree structure, that can then be used to evaluate the expression against an SG::AuxElement. This is transparent behind the ISelectionAccessor interface. The parser supports operators &&, ||, !, as well as grouping with () and any hierarchy of them. It can also handle true and false literals.

SelectionAccessorExpr doesn't support setting values, and will return either CP::selectionAccept or CP::selectionReject when getBits() is called on it.