Distribute controller operations across multiple drawers

[Bituvo]

With this pull request, the drawer controller can now distribute item operations across several drawers. When inserting, items will automatically spread to drawers with available space. When retrieving, items can be pulled from multiple drawers simultaneously. In contrast, the old behavior only allowed inserting into or extracting from one drawer slot at a time. This new behavior is toggleable via a checkbox in the drawer controller formspec (off by default).

Drawers themselves have not been changed, only the drawer controller has updated behavior.

Closes #99.

Examples

Note: my default stack size is set to 150.

Requesting items

Each drawer has 5 default:dirt.

The Luacontroller requests 15 default:dirt.

The drawers are empty and I now have 15 default:dirt. Mission successful.

Inserting items

Each drawer has 4,750 default:wood, 50 below their maximum capacity.

I insert 150 default:wood into the drawer controller.

Each drawer is now completely full, with 4800 default:wood. Mission successful.