GUI 'finishing touches'

[stackotter]

This issue is just here to act as a list of finishing touches for the GUI, most of them would be great tasks for first-time contributors. Don't feel the need to tackle them all at once, I'm just keeping the list in one place but that doesn't mean that it needs to be addressed in a single PR.

• Armour placeholder sprites in armour slots
• Offhand hotbar slot when present
• Underwater breath bubble meter
• Temporary item name when switching items in the hotbar
• Item tints in inventory (this is a bit more involved, but we have to come up with a way to decide on default tints for tinted items such as grass blocks or lily pads, at the moment they're just grey)
• Missing f3 screen info (adding new f3 info is super easy)

This is by no means an exhaustive list, if you think of anything else let me know!