Add proper panic handling

Currently, there are many "holes" in Baseview's various FFI layers, where a panic is not properly handled, crosses the FFI boundary, and ends up aborting the whole process.

Panics should be caught at least at the FFI boundary (or preferably, closer to the WindowHandler boundary), and should be resumed past the FFI boundary when the Window is interacted with host-side. (The specifics of which are TBD)