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
Remove obsolete files
Corebird.vala: code style
MainWindow: Clean up the headerbar mess
Corebird.vala: Add value to the --tweet parameter
TweetListEntry: Delete tweet on `dd`
Corebird.vala: Add null entry at the end of our cmd options

To prevent a crash on 32 bit systems, see
https://bugzilla.redhat.com/show_bug.cgi?id=1045532
Utils: Use ~/.config if $XDG_CONFIG_HOME is unset

Instead of checking $XDG_CONFIG_HOME manually, just use glib's
get_user_config_dir, but still check if ~/.corebird exists and if it
does, use that.
ui: Remove label test of hidden GtkLabels
ScrollWidget: remove old debugging output
MentionsTimeline: Don't show everything twice

This was happening whenever the user opened the page for the very first
time via the keyboard shortcut.
TweetListEntry: Reset the retweet_button at the correct position

Which is on focus-out of the entire row, not the reply_entry.
TweetListEntry: Don't hide the quick reply stuff...

... on focus-out of the reply_entry. Funnily, focusing the reply_button
and pressing return just activated the row, not the button.
TweetListEntry: Fix the rt_label once more

... just kill ImageLabel, it's causing more trouble that it's supposed
to solve.
Remove ImageLabel
MainWindow: Some whitespace fixes
schema: Fix typo