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/coreplugin/idocument.cpp

@@ -211,6 +211,16 @@ void IDocument::setTemporary(bool temporary)
     d->temporary = temporary;
 }
 
+QString IDocument::fallbackSaveAsPath() const
+{
+    return QString();
+}
+
+QString IDocument::fallbackSaveAsFileName() const
+{
+    return QString();
+}
+
 QString IDocument::mimeType() const
 {
     return d->mimeType;