broadcaster interactive plugins

> Note: requires design proposal and review before implementation

This tracks conversation about how we should support plugins that broadcasters can interact with. This poses an interesting interaction that is not consistent with the primary goals of overlayed - that is, for interactive plugins the broadcaster should be able to __click on them__. That means that we need a window host that can receive focus and therefore key and mouse events.

I think the implication here is that we must switch window modes often, or maintain yet another electron window.

Begin with the required design proposal and review rather than implementation. Define how interactive plugins should let broadcasters click and receive focus, key events, and mouse events, and compare switching window modes with maintaining another Electron window. Done means the interaction design has been reviewed and agreed upon.

Written by the indexing model from the issue text.