[remux] proposal: add process management to remux API

[raisjn]

some useful commands might be:

• stop (pause all apps)
• adopt (adopt a PID - this PID would get an entry in the switcher that persists until the PID dies)
• resume

with these commands, xdg-open style functionality could be implemented (assuming we have an image viewer application and a text viewer application and so on), s.t. an app can be launched from command line and still gets managed by the launcher. somewhere down the line, a file manager could be added which uses xdg-open under the hood.