Remix of Mellow's LLL Plus filament buffer

Differences of the remix compared to the original

• Added a filament guide to prevent filament jamming as it goes through the filament sensor bearings
• Modified lid for MMU and face-down printing.
• Added detent for main gear bearing to prevent it lifting up when changing direction
• Tweaked buffer slide for easier printing.
• Main case has been modified for easier printing (solid sides).
• Improved the buttons so they don't fall out when opening case