* Add getSelected/setSelected to core/common.js
* Migrate core to use Blockly.common.getSelected/setSelected instead of Blockly.selected
* Migrate demos to use Blockly.common.getSelected/setSelected instead of Blockly.selected
* Remove Blockly.selected
* Use variable instead of calling common.getSelected() multiple times
* Add accessor for selected on Blockly
* Add renaming record for Blockly.selected -> Blockly.common.getSelected/setSelected