Extracted TextEdit methods from EditorUtils

The methods in TextEditUtils add a dependency to refactoring which may
not be wanted in other plugins.
Adds tests for WordFinder

They come from the sdt.editor plugin in the Play support project.
Adds Region utility methods from the Play plugin
Improves doc and pre-conditions for RegionUtils
Merge pull request #802 from skyluc/issue/API-move-delete-editor

Additionnal changes needed to remove sdt.editor from the play plugin