TweetListEntry: Fix ImageDialog for inline media

The previous approach only worked for the first time the user
tried to open the dialog.
TweetListEntry: Clean up update_time_delta
Add proper time deltas to direct messages
ITwitterItem: Add update_time_delta
DeltaUpdater: Use ITwitterItems, not TweetListEntries

With this and update_time_delta in ITwitterItem, we can also add other
stuff like direct messages.
DMPage: Fix tweet sending + add entries to delta updater