Until now, the only usage of ruleset layers was where there is both a
ruleset specific and non-ruleset-specific layer present. The matching
code was making assumptions about this.
As I tried to add a new playfield layer which breaks this assumption,
non-ruleset-specifc components were not being displayed in the toolbox.
This turned out to be due to a `target` of `null` being provided due to
the weird `getTarget` matching (that happened to *just* do what we
wanted previously due to the equals implementation, but only because
there was a container without the ruleset present in the available
targets).
I've changed this to be a more appropriate lookup method, where the
target for dependency sourcing is provided separately from the ruleset
filter.
Reproduction scenario:
1. Hover a toolbox item
2. Unhover the item, but do not hover any other item (can be done by
exiting the toolbox completely to the right)
3. Come back to the item hovered in step (1)
4. The item would spontaneously contract after a second
