Reactivate a group if a contact gets readded

Fixes #723 //FREEBIE

Removes the own number from group on leaving, to receive a proper
re-added message
Finish activity before restarting it

The current activity needs to be finished before calling startActivity.
Otherwise, activities with launchMode singleTask (ConversationListActivity)
will receive a new Intent instead of getting restarted. And in response
to the new Intent, they will run onResume once again and trigger a second restart.

Fixes #1292
add version number to preferences

+ version number read from Manifest