QmlDesigner: Do not require a qmake project for designer import paths

Let's enable all build system plugins to provide this information.

Task-number: QTCREATORBUG-20810
Change-Id: I0ef953e3c2c9a2be1fc4187e93232e9a2aeacafd
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -1378,6 +1378,13 @@ QString QmakeProject::mapProFilePathToTarget(const FileName &proFilePath)
     return pro ? pro->targetInformation().target : QString();
 }
 
+QVariant QmakeProject::additionalData(Core::Id id, const Target *target) const
+{
+    if (id == "QmlDesignerImportPath")
+        return rootProjectNode()->variableValue(Variable::QmlDesignerImportPath);
+    return Project::additionalData(id, target);
+}
+
 } // namespace QmakeProjectManager
 
 #include "qmakeproject.moc"