Designer: Prepare re-implementation of "Goto Slot".

...making use of the new code model features. Move code to
separate file. Make it possible to obtain path to generated
header file from project manager.

src/plugins/projectexplorer/project.cpp

@@ -467,3 +467,9 @@ QStringList Project::frameworkPaths(const QString &) const
 {
     return QStringList();
 }
+
+QString Project::generatedUiHeader(const QString & /* formFile */) const
+{
+    return QString();
+}
+