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.

core/app.py

@@ -126,13 +126,13 @@ class DupeGuru(Broadcaster):
 
     PICTURE_CACHE_TYPE = "sqlite"  # set to 'shelve' for a ShelveCache
 
-    def __init__(self, view):
+    def __init__(self, view, portable=False):
         if view.get_default(DEBUG_MODE_PREFERENCE):
             logging.getLogger().setLevel(logging.DEBUG)
             logging.debug("Debug mode enabled")
         Broadcaster.__init__(self)
         self.view = view
-        self.appdata = desktop.special_folder_path(desktop.SpecialFolder.AppData, appname=self.NAME)
+        self.appdata = desktop.special_folder_path(desktop.SpecialFolder.AppData, appname=self.NAME, portable=portable)
         if not op.exists(self.appdata):
             os.makedirs(self.appdata)
         self.app_mode = AppMode.Standard

core/util.py

@@ -5,6 +5,8 @@
 # http://www.gnu.org/licenses/gpl-3.0.html
 
 import time
+import sys
+import os
 
 from hscommon.util import format_time_decimal
 
@@ -58,3 +60,7 @@ def fix_surrogate_encoding(s, encoding="utf-8"):
         return s.encode(encoding, "replace").decode(encoding)
     else:
         return s
+
+
+def executable_folder():
+    return os.path.dirname(os.path.abspath(sys.argv[0]))