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`.
Fix flaky template compilation unit test.

The template compiler adds a timestamp to each generated source file,
spuriously failing the test if there was more than 1 sec between the
two calls to the compiler.

We simply strip off that part of the file in the test.
Merge pull request #90 from dragos/issue/fix-flaky-test

Fix flaky template compilation unit test.
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.
Merge pull request #89 from dragos/issue/cleanup-non-editor-80

Clean-up non-editor code