Move "open" from IEditor to IDocument

For non-editor documents it currently is not used, but for editors it
makes more sense to have that on the document instead of the editor.
Most actual implementations of "open" were done in the documents already
anyhow, because it is needed for reloading.

Change-Id: I29d4df2078995cbe80172b51a9bebeecb3afad3c
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/diffeditor/diffeditor.cpp

@@ -364,14 +364,6 @@ Core::IEditor *DiffEditor::duplicate()
     return editor;
 }
 
-bool DiffEditor::open(QString *errorString,
-                      const QString &fileName,
-                      const QString &realFileName)
-{
-    Q_UNUSED(realFileName)
-    return m_document->open(errorString, fileName);
-}
-
 Core::IDocument *DiffEditor::document()
 {
     return m_document.data();