client: Convert /list #chan to UI advanced search

client: Convert /list #chan to UI advanced search

Pass "/list" parameters to ChannelListDlg when opening, allowing for
"/list #channel", <Enter> to search in a similar fashion to current
stable release.

Add documentation to the showChannelList()/related functions.

Prompt when "/list" is called without a valid network selected.  This
fixes running "/list" before any networks are selected resulting in a
channel list dialog that wouldn't do anything.