Improved event selection dialog

To select events to log Nyx uses a weird dialog, requiring users to pick letters corresponding with the events they want. To see it press 'e' on the first page...

This was ok years ago when tor had few events, but as you can see this now requires the user to type really arcane strings ('CaruF'?). We should change this to be similar to our config editor, showing a dialog like...

+------------------------------------------------------+
| BW event                                             |
| Bandwidth used by tor. This shows both the number of |
| bytes sent and received.                             |
+------------------------------------------------------+
| [ ] DEBUG                                            |
| [ ] INFO                                             |
| [X] NOTICE                                           |
| [X] WARN                                             |
| [X] ERR                                              |
+------------------------------------------------------+
| [ ] BW                                               |
| [ ] CIRC                                             |
| ... etc...                                           |
+------------------------------------------------------+

Note we'll be dropping the '--log' argument as part of this. A big part of the reason for this arcane alphabet soup was to allow users to specify events with that argument but this clunkiness is hardly worth it. Users can configure initial events in their nyxrc instead.