* Proporly handle multiple conditonal layers w/ the same target
`then-layer` values.
* Move handling to work callback, to avoid re-entrance for cascading
layers enabling other layers.
This is a generalization of the existing concept of tri-layer support
that's already well known. Essentially, a conditional-layer
configuration activates a particular layer (the then-layer) when one or
more other layers (the if-layers) are activated.
This is commonly used on ortho keyboards to activate a third "adjust"
layer while the primary two layers ("lower" and "raise") are active.
