Conversation list facelift

The conversation list looks a bit clunky and could use a redesign. Since some changes will be needed for touch/mobile anyway, this seems like a good moment to overhaul it more generally to make it a bit nicer overall.

Problems with the current layout include:

• No user avatars
• message counter is awkwardly positioned and not circular
• hover-only buttons on the left side
• heavy, somewhat glitchy-looking row divider lines
• The whole thing feels a bit cramped

I've taken a stab at cleaning up the layout a bit to address these issues. The main changes are

• action buttons are moved out of the list items to the list detail view (and possibly selection mode)
• there is an avatar
• the message counter is moved up to the top
• no divider lines

A few more states: