Keymap: Add shortcut to open Preferences

Use Ctrl + Comma. It is an expected shortcut since many
other applications use it.
Agreed during module meeting.

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -827,6 +827,8 @@ def km_screen(params):
             ("screen.screen_full_area", {"type": 'SPACE', "value": 'PRESS', "ctrl": True, "alt": True},
              {"properties": [("use_hide_panels", True)]}),
             ("screen.redo_last", {"type": 'F9', "value": 'PRESS'}, None),
+            # Preferences
+            ("screen.userpref_show", {"type": 'COMMA', "value": 'PRESS', "ctrl": True}, None),
         ])
     else:
         # Old keymap