Create MapPropertyType, change MapProperty to use it

[ljacqu]

To do

• Introduce a MapPropertyType, similar to ListPropertyType etc.
  • Make sure it can be created in different ways as in CollectionPropertyType: allow to supply a custom PropertyType<Map<String, …>> but also make it possible to just specify the PropertyType<V> valueType
  • Note: The keys of a Map are always String. This should not be overridable!
• Add unit tests

Notes

The result map property type and map property should be similar to SetPropertyType and SetProperty: make sure the entries are kept in insertion order (i.e. use LinkedHashMap) and that the default value / returned values are immutable.