Create the scaffold for the new charmstore v4 api

This branch creates the scaffold for the new charmstore v4 api and injects it into the app. It also adds the `getIconPath` method and swaps the v3 version for it. At the time of writing the current jujucharms backend has issues with returning some of the icons so this branch cannot land until those fixes are made.
ATM we only have a single one so I didn't really want to add the complexity to the method. It would be really easy to add though. Pass in a string which matches the class name and instantiate from that.