Change session cleanup mechanism

[yegorius]

I don't feel comfortable when there is a whole bash process hanging out there and waiting for the sway to exit. I think there is a better approach to this. Similar to good old pid files, we can store env variables in the user's runtime dir and use it to clean up the session.
The only significant change is that now we must exit sway by calling session.sh --exit.
Currently the config command looks like this: exec /usr/lib/sway-systemd/session.sh --exit
In order to make it look better, I suggest moving session.sh to /usr/bin/ and renaming it to sway-session so that the final command looks like this: exec sway-session --exit
WDYT?

[yegorius]

Any feedback about this?