Default account nick

Added an account setting to allow a default MUC nickname.

This can be useful when you use bookmarks from multiple devices/clients but want to be able to specify different nicknames per device.

The order of nickname preference is:
1) Bookmark nickname (if joined with bookmark) or Join nickname (if manually joined)
2) Account preference
3) Barejid of username

If the account setting is left blank the behaviour is the same as before.