Add Pointer/Touch/KeyboardHandle::with_grab() and downcasting

[YaLTeR]

Allows the compositor to inspect which grab is currently active.

Use case 1: in niri I want to scroll the workspace left and right when hovering the mouse close to output edges with a DnD grab, but not with a regular click grab.

Use case 2: avoiding a weird case with libadwaita DnD/move conflict https://gitlab.gnome.org/GNOME/gtk/-/issues/7113 by checking if the active grab is a DnD grab specifically and denying the move request in that case.

let mut is_dnd_grab = false;
pointer.with_grab(|_, grab| {
    is_dnd_grab = grab.as_any().downcast_ref::<DnDGrab<Self>>().is_some();
});

Debug impls are auto-generated with rust-analyzer (they want D: 'static).

[cmeissl]

If the main goal is to just be able check if a certain grab is active it might just make sense to add something like has_grab<T>(&self) -> bool?

[YaLTeR]

If the main goal is to just be able check if a certain grab is active it might just make sense to add something like has_grab<T>(&self) -> bool?

I figured I might as well just add the full function if a use case turns up later on.

[cmeissl]

If the main goal is to just be able check if a certain grab is active it might just make sense to add something like has_grab<T>(&self) -> bool?

I figured I might as well just add the full function if a use case turns up later on.

Not sure there is a use-case where you would need access to the dyn Trait object. I guess in most cases you either
want to know the specific grab that is active and maybe some info from it. So maybe with_grab could already take the
expected type?

(Just some idea, I am also fine with accessing the dyn Trait)

[YaLTeR]

Well, with dyn Trait you can check if a grab is active and if it's a particular grab type in one go more or less.

[Drakulix]

Seems good to me