Improved variable handling

[LaCuneta]

Right now variable names are loaded from NetLogo agents (including breeds) and globals to be available in expressions, but there is much room for improvement:

• Add some way for variables to be defined by blocks themselves and used "locally" with a block chain.
• More ways to select variable values outside of expressions - perhaps by choosers or other attributes.
• Some pre-defined way to easily set a variable value, without writing custom code for each one.
• Rudimentary manual typing of variables that NetTango knows about to restrict their placement in the blocks.