client: Sort network name case-insensitively

Set BufferViewFilter to sort case-insensitively.  This results in
network names being sorted regardless of case.  Channel/nickname
sorting is handled elsewhere.

Before:
> Alpha
> Freenode
> beta

After:
> Alpha
> beta
> Freenode

This probably matches expectations better, and hopefully doesn't
break existing workflows too much...  https://xkcd.com/1172/