Update v1.20

This is a pretty big update, with new features and a lot of bugs fixed.
You can now limit the player's tradable items inventory (if you wish).
There's also a new option to allow more control over your shops items prodction.

Version 1.20 :

• New Limited Inventory option.
• New Item production timer option.
• Customizable "stock" text.
• In some cases it was possible to buy more items than the player's limit.
• The shops could produce more items than the default max quantity.
• The shops produced items prices were constantly droping even with 0 stock.
• The prices variation could be frozen when reaching max or min value.
• There was an error in the buying price formula.
• Items customized min and max buying price values weren't correctly saved.
• Loading a save file could fail if shops weren't in numerical order.