GTK4 Astal Slider's adjustment always has page_increment set to 0

[Mabi19]

Describe the bug
The implementation of Astal4.Slider sets the underlying adjustment's page_increment property to 0 with no way to change it.
This is the property that controlls scrolling step size, and it being set to 0 causes scrolling on Astal sliders to do nothing.

To Reproduce
Create an Astal slider (for example, with <slider />) and a GtkScale (for example, with Gtk.Scale.new_with_range(Gtk.Orientation.HORIZONTAL, 0, 1, 0.01)) and display them.
Scrolling the mouse wheel on the Astal slider does nothing, but it does change the GtkScale's value (and quite dramatically in fact)

Expected behavior
The Astal slider should have a sensible default value for the page_increment property (and it should probably be configurable).

Additional context
This also causes the PgUp and PgDn keys to do nothing, as they're configured by the same property.