Parent modifiers in browsers to a base panel

[ScalarVector1]

Currently the modifier boxes used in places like the NPC and projectile spawners are directly appended to the state. That makes it difficult to check against mouse hovering. I propose we add a simple background panel element to the base Browser class to host these, that is only visible if one or more exists, similar to the filter panel.

Additionally, we could have this expand/collapse with a button similar to the filter menu