Allow starting a session with a specific LookAndFeel package

I only enabled it for wayland, might make sense for x11 too, not sure.

It does add a bunch of dependencies to startplasma-wayland, but then they're dependencies we'll be loading over time anyway.

My first thought was to do it through plasma-apply-lookandtheme but then I thought we better do it from startplasma-wayland and not rely on a separate process randomly.