callback = function(button) {
    Blockly.Variables.createVariable(button.getTargetWorkspace(), null, '');}
Construct a create variable button and push it to the xmlList.
@param {!Array.<!Element>} xmlList Array of XML block elements.
@param {Blockly.Workspace} workspace Workspace to register callback to.
@param {string} type Type of variable this is for. For example, 'LIST' or
    'VARIABLE'.