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
Merge pull request #88 from dotta/issue/extract-editor-79

Issue/extract editor 79
Removed useless call to `withPresentationCompiler`.
Small refactoring in PlayPlugin, cleaning up names.
Rely on the text editor listener for property changes.

Removed additional property change listener, relying on the framework for property
changes. As a side-effect, our editors correctly update themselves when general
editor properties have changed (like "Show whitespace").
Small refactoring in per-project preferences.
Remove AutoHashMap

There's already support for default values in the library HashMap implementation.
Removed unused `sourceFiles` map.

This may look dangerous, but it was a private map, never read. Everything seems
to work fine after removal.