Add drag-and-drop

[riverar]

This pull request adds support for drag-and-drop operations in the reactor core, including new APIs for configuring drag handlers and enabling drop targets on UI elements.

Sample:

    border
    // ...
    .allow_drop(true)
    .drag_enter({
        let set_hovering = set_hovering.clone();
        move |ctx| {
            if ctx.has_storage_items {
                set_hovering.call(true);
                DragOperation::Link
            } else {
                DragOperation::None
            }
        }
    })
    .drag_leave({
        let set_hovering = set_hovering.clone();
        move |_ctx| set_hovering.call(false)
    })
    .drag_open(|ctx| {
        if ctx.has_storage_items {
            DragOperation::Link
        } else {
            DragOperation::None
        }
    })
    .drag_drop({
        move |ctx| {
            set_hovering.call(false);
            if let Some(item) = ctx.get_storage_items().into_iter().next() {
                set_dropped_path.call(Some(item.path));
                DragOperation::Link
            } else {
                DragOperation::None
            }
        }
    })
    .into()
}

Fixes #4537.

[Copilot]

Pull request overview

Adds first-class drag-and-drop support to the windows-reactor core/WinUI backend by introducing drag/drop APIs in the DSL and wiring them to WinUI UIElement drag events, enabling apps to accept dropped data (e.g., Explorer file drops).

Changes:

• Introduces core::drag types (DragContext, DragOperation, handlers) and exposes them through the element API.
• Adds DSL modifiers for drop targets and drag event handlers, and wires them through the reconciler into the WinUI backend.
• Updates WinUI bindings and adds a minimal sample demonstrating file drop.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
crates/tools/bindings/src/reactor.txt	Adds WinUI drag/drop-related types/events to the bindings input list.
crates/tests/libs/reactor_selftest/src/bindings.rs	Updates generated selftest bindings to include drag/drop-related WinRT/WinUI APIs.
crates/samples/reactor/minimal/examples/drag_drop.rs	New sample demonstrating .allow_drop and drag/drop handlers.
crates/libs/reactor/src/winui/backend/mod.rs	Implements AllowDrop prop mapping + attaches WinUI drag event handlers and builds drag context.
crates/libs/reactor/src/dsl/modifiers.rs	Adds .allow_drop, .drag_* DSL methods and drag handler storage.
crates/libs/reactor/src/core/reconciler.rs	Applies/diffs AllowDrop and drag handlers into the backend.
crates/libs/reactor/src/core/modifiers.rs	Extends Modifiers with allow_drop + drag_handlers and updates is_empty().
crates/libs/reactor/src/core/mod.rs	Registers the new internal drag module.
crates/libs/reactor/src/core/element.rs	Re-exports drag types from core::drag for consumer use.
crates/libs/reactor/src/core/drag.rs	New core drag/drop API surface (context, operations, callbacks, handler set).
crates/libs/reactor/src/core/backend.rs	Adds Prop::AllowDrop and Backend::set_drag_handlers hook.
crates/libs/reactor/src/bindings.rs	Updates generated reactor bindings to include drag/drop-related WinRT/WinUI APIs.

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