Fix chest interaction and measure server RTT for latency meter

Chests (and lockboxes, coffers, etc.) failed to open because CMSG_LOOT
was only sent on Classic/Turtle expansions, and only when GO type was
already cached as type 3.  Fix: always send CMSG_LOOT after
CMSG_GAMEOBJ_USE (server silently ignores it for non-lootable objects).
Also broaden CMSG_GAMEOBJ_REPORT_USE to all non-mailbox WotLK GOs.

Latency meter: record pingTimestamp_ in sendPing() and compute RTT in
handlePong(); add toggleable "Show Latency Meter" checkbox in Interface
settings (saved to settings.cfg).

include/game/game_handler.hpp

@@ -1862,6 +1862,7 @@ private:
     float timeSinceLastMoveHeartbeat_ = 0.0f; // Periodic movement heartbeat to keep server position synced
     float moveHeartbeatInterval_ = 0.5f;
     uint32_t lastLatency = 0;                // Last measured latency (milliseconds)
+    std::chrono::steady_clock::time_point pingTimestamp_;  // Time CMSG_PING was sent
 
     // Player GUID and map
     uint64_t playerGuid = 0;