CheckList: Switch to QThread and add dummy checker.

src/Model/Study.py

@@ -36,6 +36,9 @@ class Study(Serializable):
     def checkers(cls):
         lst = [
             StudyNetworkReachChecker(),
+            DummyOK(),
+            DummyWARNING(),
+            DummyERROR(),
         ]
 
         return lst