[UI] MenuHandler with Pagination (v1-dev)

[phans11]

• [x] add support for creating menus
• [x] add support for pagination
• [x] add support for handling inventory closing
• [x] add support for adding items to inventory
• [x] add support for lore texts to items in inventory
• [x] add support for automatically generated inventories like the home GUI

[phans11]

MenuHandler got added to System in a first dev build. It is not finished yet and subject to change.

It's and first impression on how it would look like.