dashboard: Restore minimized experiments when trying to open them again

When the user tried to open an experiment from the explorer that
already existed, previously "nothing would happen" (focus change
without the window being restored).

artiq/dashboard/experiments.py

@@ -551,6 +551,8 @@ class ExperimentManager:
     def open_experiment(self, expurl):
         if expurl in self.open_experiments:
             dock = self.open_experiments[expurl]
+            if dock.isMinimized():
+                dock.showNormal()
             self.main_window.centralWidget().setActiveSubWindow(dock)
             return dock
         try: