Dynamic Property Registration

[mjendruk]

Changes:

• add possibility to add new property types (see examples/properties/PropertyExtension.h)
• add fromString() method to ValueProperties
• remove std::set
• add possibility to add new visit methods to visitor (see reflectionzeug/StandardPropertyVisitor.h)
• add ClassProperty and EnumProperty
• revisit VectorProperty
• add concept of PropertyCategoryVisitor in order to be able to provide default widgets e.g. for EnumProperty
• non-specialized properties inherit from ClassProperty
• add ability to check type of property with property->type() == Property<int>::stype()

[mjendruk]

works on mac and windows :shipit: