ui: prototype "hover label"-style notifications

[jackmakesthings]

A tooltip-ish text display, triggered either by click or by proximity, for snippets that don't merit a full text interface. A few use cases so far: overheard third-party conversations, and low-detail inspecting of NPCs and objects. Note - need to make sure this text doesn't fall offscreen while active.

• [x] Ability to define a popup label on an entity and set text, from json, via script or in editor
• [x] Show on click/right-click ("inspect")
• [ ] Show when nearby ("overhear") - maybe this means a trigger radius or visibility helper is needed?

[jackmakesthings]

Going to make the "overhear" feature a separate ticket since it closely ties into the structure of the npc, which I still feel like I need to clean up for better pathfinding and interactions.