Remove TextEditor::RefactoringChanges::editorForFile

This also fixes a bug with setting text cursor in InsertDefOperation in
case of split editors (where the cursor could be set in a non-active
editor on the target file).

Change-Id: I1c011386537bc88a89d4d66bec79dfe06faac3c6
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>