There are no legacy accounts anymore

If disco_muc is called without callback, user_data is None.
If user_data is None, nbxmpp does not pass it as argument to the callback

- Update name in Roster
- Update name in MessageWindow
- Update name in GroupchatControl

This reverts commit 811f3d50

This prevents one of the labels getting focus on page switch

- Use everywhere disco_muc()
- Cache results with set_last_disco_info()
- Raise muc-disco-update event after every successful disco
- Raise muc-join-failed when disco is not successful before join

- Store disco info in cache database
- Remove MucCapsCache, the DiscoInfo object supports all the methods

- On selecting the text is copied to the clipboard
- If the option is diabled CTRL + SHIFT + C can be used