Reworked chat

Reworked chat window, now it's based on Gtk.Box with custom widgets and has no limitations of Gtk.TreeView. Messages and file transfers now have their own ui files.
File transfers are now displayed and controlled in chat. 
In general, chat now looks like this http://i.imgur.com/npm9yoQ.png for me in default dark gnome 3 adwaita theme.
Changes should work normally with group chat (not sure about group action messages)