Add Channels tab to social frame and reputation change toast

Social frame now has three tabs: Friends, Ignore, and Channels. The
Channels tab lists joined channels with right-click Leave and an input
to join new channels.

Also adds a slide-in reputation change toast in the lower-right corner:
shows faction name, delta (+/-), and current standing tier (Honored,
Revered, etc.) whenever SMSG_SET_FACTION_STANDING fires a rep change.

src/game/game_handler.cpp

@@ -3517,6 +3517,7 @@ void GameHandler::handlePacket(network::Packet& packet) {
                                   delta > 0 ? "increased" : "decreased",
                                   std::abs(delta));
                     addSystemChatMessage(buf);
+                    if (repChangeCallback_) repChangeCallback_(name, delta, standing);
                 }
                 LOG_DEBUG("SMSG_SET_FACTION_STANDING: faction=", factionId, " standing=", standing);
             }