OSM plugin: area=yes is a polygon

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color : add hash support
symbolizer : is_expression helper function
text_properties : add expression_ptr's to the expression_set
cleanup
set_property_from_xml helper function
convert to expressions: text-ratio, wrap-before, rotate-displacement
more meaningful names
Fix false negative responses from python tests

Closes #2298
fix compile of svg test
Merge pull request #2294 from Vanuan/patch-1

OSM plugin: area=yes is a polygon
Merge pull request #2300 from strk/master-false-negatives

Fix false negative responses from python tests
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