text properties : displacement expr
make methods pure virtual where appropriate
pass std::string by const ref (!)
c++ style
text displacement expr : more re-factoring and various fixes
remove printing to cerr
store displacement logic in std::function and delay calc until dx/dy are evaluated (layout)
move displacement_evaluator_ initialisation
c++ style fixes
remove dead code
avoid captures in displacement_evaluator
refactor pass
remove unused member variable