Add `BlockController::getUniqueIdentifier()`

Add `BlockController::getUniqueIdentifier()`

`BlockController::getIdentifer()` returns the same result for a block
and a clipboard block that contains a reference to that block. This is
awkward for javascript and other things where you need a unique
identifier for a block
Merge pull request #1193 from mkly/add_block_controller_get_unique_identifier_method

Add `BlockController::getUniqueIdentifier()`