Block creation dialog should allow picking from a standard block

The current block creation is one of three minimal blocks. It [might be useful](https://groups.google.com/forum/#!topic/blockly/jFG9aut_ebo) to begin block creation from one of the predefined/standard Blockly block, thus allow an import feature when creating these blocks.

Would need support for some features not represent by the editor yet, like block definition extensions.