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/genericprojectmanager/genericproject.cpp

@@ -452,16 +452,6 @@ bool GenericProjectFile::save(QString *, const QString &, bool)
     return false;
 }
 
-QString GenericProjectFile::fallbackSaveAsPath() const
-{
-    return QString();
-}
-
-QString GenericProjectFile::fallbackSaveAsFileName() const
-{
-    return QString();
-}
-
 bool GenericProjectFile::isModified() const
 {
     return false;