[clients_icons] Fix icon display in roster and gc
This fixes an error preventing the plugin to load (see l. 445).
For the moment, client icons are displayed in roster and groupchat-roster only (no display in tooltip yet).
Also, I sorted the client list alphabetically (for better maintenance) and added Chatsecure, Conversations, Dino, and Pix-Art. In addition, I raised the version number above 5.8 (clients_icons plugin in 0.16 branch) to avoid confusion when updating.