Corebird.vala: Change --tweet to take a string argument

Which will be the screen_name of the account to use.
Corebird.vala: Show ComposeTweetWindow if --tweet is passed

The passed screen_name will be used as active account.
It is however planned that ComposeTweetWindow also gains the ability
to change the used account.
Corebird.vala: Rename show_tweet_window to compose_screen_name

The old name became wrong after we changed the type from bool
to string.
Corebird.vala: Don't wrongly assume --tweet