Backport mobile related patches

This backports patches related to startup splash and touch input. They're all part of GTK 4.6 and can be dropped once we have that.

They all applied cleanly, so why not have them. (this also reveals that we still have a short flicker after the splash with xdg-activation and gtk4).

EDIT: The patches to the demos are cosmetic to make startup splash work for them as well, i decided to put them in since we might ask users to run the demos to reproduce problems.