Change: rework GLOG chunk to be more like the rest

Basically it is very similar to Vehicles, where there first is
a type field, followed by data of that type. So this commit makes
it looks like how Vehicles solved that.
This removes a lot of custom "keeping track of length" stuff.

src/gamelog_internal.h

@@ -13,7 +13,7 @@
 #include "gamelog.h"
 
 /** Type of logged change */
-enum GamelogChangeType {
+enum GamelogChangeType : uint8 {
 	GLCT_MODE,        ///< Scenario editor x Game, different landscape
 	GLCT_REVISION,    ///< Changed game revision string
 	GLCT_OLDVER,      ///< Loaded from savegame without logged data