[Feature] Add cross-platform input seat support (pointer, keyboard, touch)

[zzxyb]

Is your feature request related to a problem? Please describe.

Currently, wlframe input lacks a unified abstraction for managing input devices.
On Wayland, devices such as pointer, keyboard, and touch are tied to the compositor protocol, while other platforms may require different APIs.
This makes it difficult to write renderer or UI code that can run consistently across multiple backends.

cross-platform input seat abstraction layer

Introduce a cross-platform input seat abstraction layer that provides unified interfaces for input devices:

• wlf_seat: represents an input seat containing one or more devices
• wlf_pointer: handles pointer events (motion, button, axis)
• wlf_keyboard: handles keyboard events (key press, modifiers)
• wlf_touch: handles multi-touch events (down, motion, up)
• Each backend (e.g., Wayland, headless) can implement its own input handling through a platform-specific impl structure, similar to the renderer design pattern: