A Response for background

**Is your feature request related to a problem? Please describe.**
Creating a context menu requires a `Response`, let's say I just want the context menu functionality without anything else. I would create a `CentralPanel` with `Frame::none()` and no elements and get the `Response` for a context menu from there.
If the GUI is more HUD-like, where everything outside/under the GUI is a view into some other content, for example a 2D map, that you can pan around. I would receive the inputs in the window space and translate it into some action in map space, for example creating a pin on the map with a left click. However, since I do not want to create a pin when interacting with egui's context menu I query egui with `wants_pointer_input`.
But since I used a` CentralPanel` `wants_pointer_input` will always return true (IF the code creating the `CentralPanel` runs before the pin code in a frame).

**Describe the solution you'd like**
An ability to get a `Response` from a `Context` for interaction with anything that technically isn't part of any egui element.

**Describe alternatives you've considered**
I wasn't sure if this should be more of a feature request or a bug in a vein of "Context always wants input with CentralPanel" or "FrameState doesn't respect Frame::none".
As for other solutions, the order of the `wants_pointer_input` and `show`ing the `CentralPanel` matters, but I still think it should be fixed as in multithreaded/dynamically ordered frame code this may be a race condition or a problem that appears at random on different runs of the program.

Start by reading the Context, CentralPanel, and Response APIs involved in pointer-input handling. Compare the current CentralPanel behavior with the requested context-level interaction response, then define and validate an approach that lets callers handle context-menu input without treating the empty panel as the underlying map interaction target.

Written by the indexing model from the issue text.