Add portable mode

If settings.ini is present next to the executable, will run in portable mode.
This results in settings, data, and cache all being in same folder as dupeGuru.

qt/app.py

@@ -52,7 +52,7 @@ class DupeGuru(QObject):
         # Enable tabs instead of separate floating windows for each dialog
         # Could be passed as an argument to this class if we wanted
         self.use_tabs = True
-        self.model = DupeGuruModel(view=self)
+        self.model = DupeGuruModel(view=self, portable=self.prefs.portable)
         self._setup()
 
     # --- Private