[feature] 60 slots backpack

[FrogSkyWater]

Expand the 48 slots with a second "buyable" backpack with 60 slots

many things broken as per email chat, backlog :

• dialogues (first one showing 48, the other one showing 60) during buying process
• show separate backpack .png for the 48 and 60 bpack
• in-game commands seems broken
• edit the config.json generation to allow user to set the price of both backpacks
• Left-mouse click in any slot of the 5th row closes the interface. fyi 'More Backpack upgrades' mod has the same bug, i tried to resize UI but unsuccessful (it's a minor bug)