WIP: MAINT: refactor retrieve properties (move backend-logic to backend)

src/main/java/org/caosdb/server/database/DatabaseUtils.java

...backend/implementation/MySQL/MySQLRetrieveProperties.java

...r/database/backend/interfaces/RetrievePropertiesImpl.java

...rver/database/backend/transaction/RetrieveProperties.java

.../java/org/caosdb/server/database/proto/ProtoProperty.java

src/main/java/org/caosdb/server/entity/wrapper/Property.java

...org/caosdb/server/entity/xml/EntityToElementStrategy.java

...g/caosdb/server/entity/xml/PropertyToElementStrategy.java

src/test/java/org/caosdb/server/database/InsertTest.java