Stubs for abstract members

Stubs for abstract members implementation

This pull request adds stubs generation for available abstract defs
Usage: select abstract def for generation from quick fix menu (invoke menu on required class) 

Current version main limitations:
1) There is no stub generation for abstract val/vars
2) Type parameters for defs are not generated
Issues 1), 2) require changes to scala-refactoring's [AddMethod](https://github.com/scala-ide/scala-refactoring/blob/master/org.scala-refactoring.library/src/main/scala/scala/tools/refactoring/implementations/AddMethod.scala).