Refactor the plugin to use the lexical variable mixin from App Inventor

[mark-friedman]

• [ ] Remove the mixin defined methods from each block.
• [ ] Add the methods required by the mixin to each block.
• [ ] Mix the mixin into each block. Test the result.
• [ ] For each block.foo(...) method defined in the mixin create a new foo(block, ...) function.
• [ ] Change the code in field_lexical_variable.js (and any other code) that calls a block.foo(...) method defined in the mixin to call the new foo(block, ...) function instead. Test the result.
• [ ] Remove the mixin

[mark-friedman]

Note that once this is done, we can then do following to App Inventor (similar to the above):

• [ ] Remove the mixin defined methods from each block.
• [ ] Add the methods required by the mixin to each block.
• [ ] Remove the mixin