[NONE] Implement DeleteOnClick via hover handles.

The FXHoverBehavior is adjusted to generate handles:
 - handles are only generated when the mouse is not moved for a specified creation delay
 - when handles are generated, feedback and handles will not be removed immediately, but
   only if the host & handles are not hovered for a specified removal delay
[NONE] Activate/deactivate NodeContentPart when expanding/pruning, respectively.
[NONE] Only generate hover handles when the mouse does not move during the creation delay.
[NONE] Remove unused local variable.