TweetListEntry: Set correct link handler of rt_label

... woops.
Utils: Remove unneeded catch clause
ImageLabel: Fix the label's size

See issue #64
Add a TweetListBox widget

... we us to unify the placeholder stuff which now also has a way to
indicate that simply no results have been found.
Relevant for issue #57
ProfilePage: Use a TweetListBox

So we can easily indicate missing tweets if the user has not yet tweeted
anything.
Utils: Use $XDG_CONFIG_HOME in user_file

See issue #65
tweet-list-entry: Dont show the conversation_image no show_all