Add SkinModifier

[necropotame]

Skin modifier is a way to customize the skin of a player. The definition of the skin modifier is stored in the mod file with this structure:

struct CModAPI_ModItem_SkinModifier
{
//Replace "Part" by each part of the skin
int m_PartFlag; // REPLACED | CUSTOMCOLOR | EXTERNALFILE
char* m_PartSkin; //Name of the skin file
int m_PartColor;
...
int m_HookStyle; //A line style ID
};

Then, if the a skin modifier is attached to a player (just an ID), the client change part of the skins that a has the flag REPLACED. The hook is also replaced by m_HookStyle if m_HookStyle >= 0.