Add ModStorageAPI to client side modding (#5396)

mod storage is located into user_path / client / mod_storage

src/unittest/test.cpp

@@ -65,6 +65,9 @@ public:
 		return testmodspec;
 	}
 	virtual const ModSpec* getModSpec(const std::string &modname) const { return NULL; }
+	virtual std::string getModStoragePath() const { return "."; }
+	virtual bool registerModStorage(ModMetadata *meta) { return true; }
+	virtual void unregisterModStorage(const std::string &name) {}
 
 private:
 	IItemDefManager *m_itemdef;