Adding DefaultBlocks, constants related to the default/playground blocks

[AnmAtAnm]

• Adding DefaultBlocks, a class to store constants related to the default/playground blocks.
• Adding support for Mutators in the AbstractBlocklyActivity and BlocklyActivityHelper.
• Minor tweaks to DevTests toolbox (previously, "Some blocks" and "Outputs" were identical, looking like a bug).

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This change is 

[RoboErikG]

couple missing TODOs

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Review status: 0 of 9 files reviewed at latest revision, 2 unresolved discussions.

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blocklylib-core/src/main/java/com/google/blockly/model/DefaultBlocks.java, line 64 at r1 (raw file):

    public static Map<String, BlockExtension> getExtensions() {
        if (DEFAULT_EXTENSIONS == null) {
            Map<String, BlockExtension> temp = new ArrayMap<>();

TODO?

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blocklylib-core/src/main/java/com/google/blockly/model/DefaultBlocks.java, line 77 at r1 (raw file):

    public static Map<String, Mutator.Factory> getMutators() {
        if (DEFAULT_MUTATORS == null) {
            Map<String, Mutator.Factory> temp = new ArrayMap<>();

Ditto

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