Make "write tweet" window multi-account capable

[HeikoAdams]

Currently you can only write tweets for the account in the current corebird window. But if you configured several accounts it would be a huge usability improvement if you could write tweets for all configured accounts regardless which one is opened in the current window.

[baedert]

I wanted this more for the --tweet command line option so one can pass a default screen name but still change that.