Adds XSource prompt on Classpath validation & StatusHandler support throughout

This is an extension of https://github.com/scala-ide/scala-ide/pull/678

How to test this UI-based PR:
- `MissingScalaRequirementHandler` : open scala-compiler
- `ClasspathErrorPromptStatusHandler` : see [this](https://github.com/scala-ide/scala-ide/pull/678#issuecomment-41929816)
- `ClasspathErrorPromptStatusHandler` : put a breakpoint in `ScalaLaunchDelegate` / `ClasspathErrorPromptStatusHandler`, Debug as [Eclipse Applicaiton with Weaving](http://scala-ide.org/docs/dev/setup/setup.html), in the opening Eclipse, create a Run Configuration for any Scala project without a `main()` method and run it.