Unify monsters.xml back

You might want to discuss with other developers (e.g. HoraK, Hocus, Reid, etc.) about this.

The only advantage of keeping them separate is official mods which modify items, or player mods to modify items.

The advantage of keeping in one file is making easier to search, finding unused IDs (they are marked, search for "free" in the new file), and slightly faster M+ startup time as it doesn't need to make a lot of I/O operations.

I might have overlooked advantages and disadvantages on both sides. In any case, ManaPlus only reads a single file, so due to the nature of _includes.xml, merge conflicts may happen in both methods.