Items & Save system overhauls

[TML233]

A complete overhaul of item system and flag system.

Items are now implemented with structs. Item IDs are saved in inventories. You can have multiple inventories, e.g., item inventory, cellphone inventory, each of the boxes.

Flag system is replaced by Storage system. For each storage, you can have storage zones. StorageZoneStruct holds a struct that you put data there. StorageZoneInventories saves and load all inventories registered. Save data are encoded in JSON format.

Flag_Custom is replaced with Player_CustomInitialData.

Many minor code enhancements are also present.   Closes #123 Closes #124