Don't (re-)connect the document to the compilation unit on completion

The document gets attached to the compilation unit when it's created, hence
there should be no need to re-connect it during code completion.
Extracted common editor-related functionality in separate bundle

* Created a new bundle: ``org.scala-ide.sdt.editor``. This bundle will
  eventually be moved in the Scala IDE codebase.

* The rationale I followed is to extract only the logic that was clearly
  duplicated in both the Play2 template editor and the Worksheet's Script
  editor.

Fix #79