Adds Block.prototype.mixin() and Blockly.Extensions.registerMixin().
This adds support for a common use pattern in extensions, and adds
error checking to avoid future incompatibilities.
* Beginnings of a JSON block definition unit test set.
* Dispose of unit test workspaces and blocks in finally blocks.
* Clarify JSON error message by echoing arg notation.
Replaces old colour block definitions with a Blockly.defineBlocksWithJsonArray(..) call. Generator unit tests continue to load and pass, signifying compatibility with prior block definitions.
Replaces extension 'math_number_tooltip' with the reusable 'parent_tooltip_when_inline' extension, also used by colour_picker. Includes tests.
Adding support for extensions, functions that can assist with loading blocks, much like init functions, but that can be referenced from JSON definitions. This allows JSON definitions to define dynamic blocks such as onchange handlers and mutators.
Rewrote math_number as an example pure JSON block.
