Refactor compositors to allow state-based implementation of canConnect() / connect() etc.

• Currently, many methods of compositors that are meant to be overridden (such as canConnect(), canConnectInterally(), connect()) are stateless, i.e. do not into account the current source and target model elements.
• Hence, the following base classes have potentially dangerous work-arounds using protected fields that are only valid in a given call scope
  • HierarchicElementConnectorConnectionCompositorBase
    • source
    • target
    • updateSourceTarget()
  • PlatformElementConnectionCompositorBase
    • source
    • target
    • updateSourceTarget()
• Instead of using fields, it would be much safer to extend the interface of the respective methods (connect(), canConnect()) accordingly

(from redmine: issue id 2234, created on 2015-01-27)