Implement undo snackbar when deleting messages by swiping.

When swiping away a message it is only removed from the local message
lists and a snackbar appears. When the snackbar is dismissed or the user
deletes another message the delete request is sent to the server. If the
user presses "undo" on the snackbar the message is reinserted into the
local lists at its previous position.

app/src/main/res/values/strings.xml

@@ -25,6 +25,8 @@
     <string name="grouped_message">Received %d messages while being disconnected</string>
     <string name="delete_all">Delete all</string>
     <string name="delete_logs">Delete logs</string>
+    <string name="snackbar_deleted">Message deleted</string>
+    <string name="snackbar_undo">Undo</string>
     <string name="all_messages">All Messages</string>
     <string name="logs">Logs</string>
     <string name="message_image_desc">The image of a message</string>