ImageDialog: Fix valac warning
MainWindow/switch_page: If cur_page == page_id, just call on_join

... it won't be accessible via the back button, but it's still better
than our current approach of just ignoring it.
ComposeWindow: Make the text view not accept tabs

... also, add Ctrl+Return as a shortcut to send the tweet.
style: Make the window draggable on the sidebar

OH YES.
Merge branch 'master' of https://github.com/baedert/corebird