* Added ElementHints.KEY_HIDDEN for stating that an element should be considered...

* Added ElementHints.KEY_HIDDEN for stating that an element should be considered hidden and no it should not:
** show up in any diagram/element measurements
** be involved in picking or any other interaction
** be rendered
* Changed InternalSize.getBounds specification to allow implementations to differentiate between cases where an element has no area (is infinitesimally small) and that an element does not exist visually (has no bounds). This also helps in the implementation of proper diagram element measurements when parts of the diagram are hidden.

fixes #2091

git-svn-id: https://www.simantics.org/svn/simantics/sysdyn/trunk@19757 ac1ea38d-2e2b-0410-8846-a27921b304fc