Locator framework

Add a new engine locator framework for searching for engines on the
disk. Changes the implementation of the CordovaEngineProvider so that it
uses the framework to locate the available engines. Introduces
PlatformLibrary to keep track of the locations together with the
HybridMobileEngine instances so that they can be located anywhere.
bug JBIDE-16408 - Use hybrid mobile engines from local directories

Adds a search feature to the AvailableCordovaEnginesSection to look for
Cordova engines on a given local directory. Also improves the UI so that
the locations of the engines are displayed