Make isValidChannel() static and check user input when adding channels.

Shows a warning dialog when adding which informs the user the channels
won't be joined and added.
Huge changes

- Saving window position and size
- Make Settings a singleton
- Handling of selected connection/channel
- Userlist now displaying users in selected channel
- Treat incoming bytes as utf-8
- Temporary remove partial SSL code
- Added a buffer for messages per channel (but not used yet)
- Basic alert and notification popup
- Optional tray icon (default is true) with menu
- Nickname completion