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/qmakeprojectmanager/qmakenodes.cpp

@@ -280,16 +280,6 @@ bool QmakePriFile::save(QString *errorString, const QString &fileName, bool auto
     return false;
 }
 
-QString QmakePriFile::fallbackSaveAsPath() const
-{
-    return QString();
-}
-
-QString QmakePriFile::fallbackSaveAsFileName() const
-{
-    return QString();
-}
-
 bool QmakePriFile::isModified() const
 {
     return false;