Add default implementation for IDocument::fallbackSaveAs(Path|FileName)

The methods are only relevant for documents without a filePath, and
there was a mix of different irrelevant implementations present in
subclasses.

Change-Id: I4f57d306e5ddd913974cfe6ed0b4db062eb907a1
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/pythoneditor/pythoneditorplugin.cpp

@@ -293,9 +293,6 @@ public:
         return false;
     }
 
-    QString fallbackSaveAsPath() const override { return QString(); }
-    QString fallbackSaveAsFileName() const override { return QString(); }
-
     bool isModified() const override { return false; }
     bool isSaveAsAllowed() const override { return false; }