New Variant type

Things changed:
* replace old Variant type by new one
* add options and flags to properties by using Variant class
* remove all GUI related setters and getters from properties and replace them by options and flags __(The implicit interface of e.g., editors is documented in the related class comments [see here](https://github.com/hpicgs/libzeug/blob/d3df6deec36553bb29936dde3706231ab4bc4f2f/source/propertyguizeug/include/propertyguizeug/PropertyModel.h))__
* add methods to convert from and to Variant for ValueProperties