Fix a bug and add an improvement

Here are two small commits:

 * the first one allows modifying the item margin from XML
 * the second one fixes a bug: when the activity is recreated after being killed for memory reasons, there was a crash.

The second bullet covers the following use case: if you load the activity with the SGV, open a sub-activity, and switch to another app. Back from the main app after some time (so that it is killed by Android), you're on the sub-activity. Press the back button, boom!