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
Ensure read-only transactions

Closes #2142
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
fix path_move_dx
make add_expressions pure virtual in base class
move set_property_from_xml into separate header for re-use
make set_property_from_xml more generic
(e.g work with symbolizer_base::value_type and optional<symbolizer_base::value_type>)
add has_attribute(std::string const&) member function
expressions in text/formatting/layout (work-in-progress)
orientation -> symbolizer_base::value_type
use 2 spaces identation in xml output
prefer c++11 range based loops for readability
add property_serializer impl