Support mutatedBlocks in toolbox properly (#4287)
Support mutatedBlocks in toolbox properly (#4287)

* mutated blocks show multiple blocks in the toolbox. Support returning a number of blocks instead of one when calling getBlocksXML

* Minor fix.

Microsoft/pxt,switchinnovations/pxt,playi/pxt,Microsoft/pxt,playi/pxt,switchinnovations/pxt,Microsoft/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt,switch-education/pxt,switchinnovations/pxt,playi/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt