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/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]))