* Adds message references to message string interpolation, in the form of %{BKY_STRING}.
* Re-adding CONTROLS_IFELSE block using the new syntax, referencing to CONTROL_IF equivalents.
Adding controls_ifelse, an if/else block that is loaded from JSON and does not use mutators. This gives "else" capability to Android & iOS implementations, which don't support JavaScript mutators.
Added this block to the playground simple toolbox and all generators.