gtk4: add scroller widget

[kotontrion]

This PR introduces Astal.Scroller, a new GTK widget that implements automatic scrolling. Which is useful for example titles and artists in an mpris widget.

• Scrolling Behavior: Supports ALTERNATE (back-and-forth) and SLIDE (continuous loop) scrolling.
• speed: Defines scrolling speed (pixels per second).
• direction_change_delay: Delay when changing direction (if ALTERNATE mode is enabled).
• orientation: Determines horizontal or vertical scrolling.
• propagate_child_size: Determines whether the scroller should adjust based on its child's size.

[Curve]

Sorry for asking, but are there any news on this :)?