add a uniform invocation method for all extraction refactorings

Adds the “Extract… Alt + Cmd + E” to the refactoring menu which
gives access to the following refactorings:

- Extract Value
- Extract Method
- Extract Parameter
- Extract Extractor

After invocation “Extract…” collects all applicable extractions based
on the currently selected code. In order to reduce the number of
proposed extractions, “Extract…” analyses the selected code for side
effects. If it assumes the code triggers side effects, it prefers
method extractions over value extractions if the according target scope
allows both.

Furthermore, this patch integrates the new extraction refactorings in the quick assist menu. The additional quick assist processor uses the identical implementation as the “Extract…” command.

Known limitations:
- Detection of side effects currently uses a relatively simple
heuristic that may lead to impracticable proposals

Conflicts:
	org.scala-ide.sdt.core/plugin.xml