Use UTF-8 instead of narrow

Use wide_to_utf8 and utf8_to_wide instead of wide_to_narrow and narrow_to_wide at almost all places.
Only exceptions: test functions for narrow conversion, and chat, which is done in a separate commit.

src/network/clientpackethandler.cpp

@@ -228,7 +228,7 @@ void Client::handleCommand_AccessDenied(NetworkPacket* pkt)
 		if (pkt->getSize() >= 2) {
 			std::wstring wide_reason;
 			*pkt >> wide_reason;
-			m_access_denied_reason = wide_to_narrow(wide_reason);
+			m_access_denied_reason = wide_to_utf8(wide_reason);
 		}
 	}
 }