QmlJS: Allow setting import paths in Qt4Projects using QML_IMPORT_PATH.

Introduced new Creator-specific qmake variable QML_IMPORT_PATH that
lists the import paths required by the QmlJS engine.

Done-with: Alessandro Portale

src/plugins/qt4projectmanager/qt4nodes.h

@@ -94,7 +94,8 @@ enum Qt4Variable {
     PkgConfigVar,
     PrecompiledHeaderVar,
     LibDirectoriesVar,
-    ConfigVar
+    ConfigVar,
+    QmlImportPathVar
 };
 
 class Qt4PriFileNode;