Show a dialog if the compose button is pressed with no contacts/groups.

briar-android/res/values/strings.xml

...droid/src/net/sf/briar/android/BriarFragmentActivity.java

...id/src/net/sf/briar/android/groups/GroupListActivity.java

...droid/src/net/sf/briar/android/groups/NoGroupsDialog.java

...et/sf/briar/android/groups/WriteGroupMessageActivity.java

...t/sf/briar/android/messages/ConversationListActivity.java

...d/src/net/sf/briar/android/messages/NoContactsDialog.java

...f/briar/android/messages/WritePrivateMessageActivity.java