Support for drop to frame

Support for drop to frame button for the Scala IDE debugger. I think, it's not needed to explain what's that, as it's a well-known feature from the Eclipse's debugger for Java.

An implementation is really simple - except some checks whether this operation can be performed it's basically: invoke threadRefrence.popFrames(frame) and stepInto using the stack frame just below this one used when invoking popFrames. The whole UI support is handled by Eclipse as ScalaStackFrame now extends IDropToFrame (Eclipse just does some type casting and then does everything, what we need, when it sees that it got IDropToFrame instance).

Note:
- it can skip some frames as StepInto does this intentionally
- there's no Hot Code Replace (HCR) yet - I'll try to add it soon (I mean it's a separate feature)

Lastly, the below disclaimer is required by the lawyers:

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