Bump protocol to 26

This was needed due to the TileDef serialisation changes. Originally it has been planned
to also add utf-8 based chat to 26, but chat changes aren't final yet, so they are done
in one change, after the release, and not two small ones, causing us having to be compliant
to three versions of the packet.

Author: est31

src/network/networkprotocol.h

@@ -134,7 +134,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 		Add TileDef tileable_horizontal, tileable_vertical flags
 */
 
-#define LATEST_PROTOCOL_VERSION 25
+#define LATEST_PROTOCOL_VERSION 26
 
 // Server's supported network protocol range
 #define SERVER_PROTOCOL_VERSION_MIN 13