Add target ids

[cboulanger]

At the moment, targets can only be differentiated by "type", i.e. a target is either "source" or "build", which is a shortcut for the underlying implementation class. In order to have specialized targets, one needs to create separate compile-XXX.json files which contain the specializations. This leads to a lot of code duplication, and is not ideal.

This problem can be solved by introducing “target-ids”. Currently the —target= specifies a type, but if it actually specified an ID, the target definition could be:

{ 
  type: “build”, 
  id: “deploy-debug”,
  // …
}

In order to be backwards-compatible, the IDs of “source” and ”build” will be made reserved IDs, in that if you have a id: “source” then it must be a type: “source”- and also the default id is the type. This would allow (eg) qx deploy —target=deploy-debug, but all the previous syntax would still work.

The implementation involves creating a seperate output directory for each target-id. There’s already some code that selects the correct target based on application type etc (ie you can have multiple type: “build” already, one for browser, another for node, etc).