Add input event callback to DisplayServerHeadless

[mihe]

This adds input capabilities to DisplayServerHeadless. by simply adding an input event callback to it, in pretty much the exact same way as how DisplayServerMock does it, while also removing the now redundant one from DisplayServerMock.

This could perhaps be considered a mildly controversial change, seeing as how input handling is tied to windows in the eyes of the display server API, but in the interest of sparking a discussion about it, here it is.

The motivation for this isn't too complicated: When dealing with things like unit tests that run on a headless CI, it can be desirable to manually inject input events (with something like Input.parse_input_event) in order to test the input code paths, or perhaps simulate an actual user more accurately.

This is technically possible already by relying on Viewport.push_input instead, but there you run into the problem that's mentioned in its documentation, namely that input emulation (like input_devices/pointing/emulate_touch_from_mouse) doesn't exist in that context, leaving you to having to recreate/copy this emulation into your own code instead, which is somewhat unfortunate and potentially brittle. This emulation also can't be added to Viewport.push_input since that's in turn used by Input.parse_input_event, so you'd end up not only with double emulation, but emulated events themselves triggering further emulation.

The much simpler solution to this seems to be to just add an input event callback to DisplayServerHeadless, allowing you to use Input.parse_input_event and its emulation just fine.

[Calinou]

Does this PR work with #92198?

[bruvzg]

Does this PR work with #92198?

#92198 simulate native OS events on the OS API level, therefore it needs real DisplayServer which can handle OS events to work. It won't work with DisplayServerHeadless, or interfere with this PR in any way.

[akien-mga]

Looks good to me.

This is pretty important for unit testing so I'll merge despite the feature freeze for 4.3, and consider it for a 4.2.3 cherry-pick.

[akien-mga]

Thanks!