* Propagate the visible state when blocks connect
This fixes#1967.
In rendered connections when connecting:
- If the superior connection is hidden this hides the newly connected block.
- If the superior connection isn't hidden it makes sure the block is visible.
In rendered connections when disconnecting:
- If the superior connection is hidden, make the disconnected block stack visible.
TODO before review:
- write tests.
- update collapsed message
* Add missing overrides
* Add tests for hidden connections and fix a bug while disposing
