Add -s option to start any application to sway command

[stefonarch]

Description:

Having the possibility to start sway with sway -s lxqt-session would be nice. This is possible with labwc 0.7.2 and kwin_wayland.

We plan to support all wlroots-based compositors but adding a command to their autostart settings by our startup script is not ok, and 2 different configuration files isn't that user friendly too.

[Vladimir-csp]

Is it the right way to do it though? Shouldn't a session manager run the compositor and not the other way around? (Asking for a friend who wrote a session manager)