components: implement popup menu

This is a simple popup menu within a FullScreenOverlay, which means that no touch event is delivered to items below until the menu gets closed (by tapping outside).
The menu adjusts its position (at the moment only its y component) out of the y coordinate of the item to which it is related: if less than half of the screen, the popup will spawn below the item, above it otherwise. If no item is provided, the menu will spawn at the centre of the screen.
In the future we may think about adapting this behaviour for the x component as well.

The last commit shows a use case for Halcyon.