Feature/#501 refactoring resource actions

* Right click menu options related to file view are now extracted into a command pattern classes. 
* The right click menu is generated in code and no longer in a xib: Work is the same, maybe even less to .coordinate actors, and abit less stuff in the godly MainMenu.xib.
* [Some meta documentation on this.](https://github.com/spritebuilder/SpriteBuilder/wiki/ResourceCommands)
* Fixes a bug for export packages and the right click menu: menu item would be appended over and over again upon clicking.