Created interface 'ListenerHandle' (still undocumented).
[!] New class 'ExecuteWhen' builds 'ExecuteAlwaysWhen' and 'ExecuteOnceWhen'. Constructor visibility reduced to package. Also included wildcards were possible and improved comments.

ATTENTION: Unfortunately this revision does not compile! I screwed things up... :(
[!] Adapted the WebView hyperlink listening to implement 'ListenerHandle'.

ATTENTION: This revision makes [b6c35e5] compile again.
Adapted control property listening to implement 'ListenerHandle'.
Adapted nested listener to implement 'ListenerHandle' (undocumented).
Defined contract for 'ListenerHandle'.
'ControlPropertyListenerHandle' implements 'ListenerHandle' contract. This is now fully documented and tested.
'WebViewHyperlinkListenerHandle' implements 'ListenerHandle' contract.
Nested listeners implement 'ListenerHandle' contract. This is now fully documented and tested.
Implemented a generic 'ListenerHandle'.
Implemented a builder for 'GenericListenerHandle'.
Utility class for listener handles.
Listener handles now have their own subpackage in 'listener': 'org.codefx.libfx.listener.handle'.
Factory class 'ListenerHandles' creates handles for 'Observable' and 'ObservableValue'.
Factory class 'ListenerHandles' creates handles for observable collections.
Documented package 'org.codefx.libfx.listener.handle'.
Started to implement a 'ListenerHandleDemo'. It currently covers the construction of handles.
Renamed 'ListenerHandles.create' to 'ListenerHandles.createAttached' to better get across that the listener is added to the observable.
'ListenerHandleDemo' now also shows how to attach and detach a listener.
Merged branch 'feature/listenerhandle' into develop.