Add support for whole layer rarities

[Montspy]

Whole layers can now have a non-zero chance to be empty (i.e. skipped).
Metadata is generated with missing property or attribute where a layer is skipped.
A field for layer rarity has been added in the UI, and UI has been reworked to improve readability.

Closes #98 as it implements the weights_total layer key and the backward compatibility logic

Metadata missing Eyes layer:

{
    "name": "A Cool Collection #004",
    "description": "Very cool collection, much wow!",
    "royalty_percentage": 0,
    "id": 4,
    "attributes": [
        {
            "trait_type": "Colors",
            "value": "Blue"
        },
        {
            "trait_type": "Landscape",
            "value": "Mountain"
        },
        {
            "trait_type": "Body",
            "value": "Tri"
        }
    ],
    "properties": {
        "Colors": "Blue",
        "Landscape": "Mountain",
        "Body": "Tri"
    },
    "royalty_address": "0xe6bedb20f57e694f0c4e28946829bcc4dbdfc4a5",
    "artist": "Monty!",
    "image": "ipfs://Qmf4JrFepnJCsnHTsezxjftLLLNr5oqHdoEnAeHpahLff1",
    "animation_url": "ipfs://Qmf4JrFepnJCsnHTsezxjftLLLNr5oqHdoEnAeHpahLff1"
}

Generation statistics also accounts for missing layers:
gen_stats.json for 10 images (note per layer totals <= 10)

{
    "Colors": {
        "Red": 6,
        "Green": 2,
        "Blue": 1
    },
    "Landscape": {
        "Island": 2,
        "Mountain": 5,
        "Tunnel": 1
    },
    "Body": {
        "Tri": 2,
        "Squ": 3,
        "Cir": 5
    },
    "Eyes": {
        "Dash": 0,
        "Dot": 3,
        "Cross": 1,
        "Sunset": 1,
        "DotAgain": 0
    },
    "seed": "toast"
}