Add dictionary variables and related blocks

[tansly]

See also FBDY/bb-vm#50.

TODO:

• See the commented out lines:

  bb-blocks/core/data_category.js

  Lines 103 to 116 in 9042160

  	Blockly.DataCategory.addAddToDict(xmlList, firstVariable);
  	Blockly.DataCategory.addSep(xmlList);
  	// Blockly.DataCategory.addDeleteOfList(xmlList, firstVariable);
  	// Blockly.DataCategory.addDeleteAllOfList(xmlList, firstVariable);
  	// Blockly.DataCategory.addInsertAtList(xmlList, firstVariable);
  	// Blockly.DataCategory.addReplaceItemOfList(xmlList, firstVariable);
  	Blockly.DataCategory.addSep(xmlList);
  	Blockly.DataCategory.addItemOfDict(xmlList, firstVariable);
  	// Blockly.DataCategory.addItemNumberOfList(xmlList, firstVariable);
  	// Blockly.DataCategory.addLengthOfList(xmlList, firstVariable);
  	// Blockly.DataCategory.addListContainsItem(xmlList, firstVariable);
  	Blockly.DataCategory.addSep(xmlList);
  	Blockly.DataCategory.addShowDict(xmlList, firstVariable);
  	Blockly.DataCategory.addHideDict(xmlList, firstVariable);

  
  Determine which of these operations are applicable to dictionaries and implement the ones which are. -> Implemented the ones that I think are suitable.
• Check this out:

  bb-blocks/blocks_vertical/data.js

  Lines 736 to 740 in 9042160

  	/**
  	* XXX: This is copied from the mixin for the list context menu.
  	* I've modified it without paying much attention so beware.
  	* TODO: Inspect this in detail.
  	* Make sure the stuff about variable renaming, deletion etc. work properly.

  
  I vaguely remember testing this stuff but I'm not sure. Remove the comments when sure.
• Review block messages for understandability.
• for key in dict block?
• Polish for key in dict block, fix the non-existent field warning -- will deal with this later

[tansly]

Now, there is the long work of adding related blocks one by one... Then comes the VM work for their execution at FBDY/bb-vm#51.

[tansly]

At this point it seems like a good idea to work on FBDY/bb-vm#51 in parallel to this. At least testing the add/get blocks would be nice.