[cuegui] Prevent UI freeze during file preview by implementing subpro…

…cess.Popen for non-forking viewer applications (AcademySoftwareFoundation#1576)

This is a small fix to not lock the UI when using preview in CueGUI and when using software that does not detach itself from the console.

It changes it from using `subprocess.check_call` to `subprocess.Popen`

Fixes AcademySoftwareFoundation#1568

cuegui/cuegui/Utils.py

@@ -666,7 +666,8 @@ def launchViewerUsingPaths(paths, actionText, test_mode=False):
     if not test_mode:
         print(msg)
         try:
-            subprocess.check_call(cmd.split())
+            # pylint: disable=consider-using-with
+            subprocess.Popen(cmd.split())
         except subprocess.CalledProcessError as e:
             showErrorMessageBox(str(e), title='Error running Viewer command')
         except Exception as e: