Core: Add methods to retrieve contents from IDocument

This is very helpful for dynamic extra compiler support (e.g. uic,
flex, ...).

Change-Id: I5a49b0b39a0f1fbc7834701d60a68ef441172969
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/idocument.cpp

@@ -150,6 +150,15 @@ IDocument::OpenResult IDocument::open(QString *errorString, const QString &fileN
     return OpenResult::CannotHandle;
 }
 
+/*!
+ * Returns the current contents of the document. The base implementation returns an empty
+ * QByteArray.
+ */
+QByteArray IDocument::contents() const
+{
+    return QByteArray();
+}
+
 /*!
     Used for example by EditorManager::openEditorWithContents() to set the contents
     of this document.