Core: generic property handling methods (#472).

The DeepaMehta Core service provides 3 new methods for handling generic properties:
    - getProperty()
    - setProperty()
    - hasProperty()

AccessControl service:
    - getACL() is new method

BREAKING CHANGES

DeepaMehta Core service:
    - 6 methods are dropped:
        - getACL()
        - setACL()
        - getCreator()
        - setCreator()
        - getOwner()
        - setOwner()
      Use the AccessControl service instead

AccessControl:
    - 4 classes have moved from package de.deepamehta.core.service.accesscontrol to
                                        de.deepamehta.plugins.accesscontrol.model:
        - AccessControlList
        - ACLEntry
        - Operation
        - UserRole
      You must adapt your imports.

See ticket 472.