python/ui: whole layer rarity

[Montspy]

We should allow for the user to set a percentage of chance to not generate a whole layer.
Currently users have to create an empty trait with a fully transparent image.
This isn't ideal and this "Empty" variation still shows up in the attributes and properties.

Adding our previous discussion here:

Add a weights_total key. The chance of the layer being empty is then weights_total - sum(weights). If the weights_total key doesn't exist we assume weights_total = sum(weights) (this is to be compatible with old collections using named rarities that don't add up to 100% (Common, Rare, Legendary)