Add NSM support

[sub26nico]

Hi, it would be a very useful feature to add NSM support to use linux-show-player with other JACK applications, all apps could be managed by a session manager (RaySession, New Session Manager).
More information about NSM here.

[clemradio]

Any JACK compatibility is always welcome

[s0600204]

For future reference, here's the API documentation:

• for Non Session Manager: https://non.tuxfamily.org/nsm/API.html
• for New Session Manager: https://new-session-manager.jackaudio.org/api/index.html

(The latter NSM is a fork of the former NSM, and claims its API is 100% backwards compatible. Thus the two documentation sources should describe - and the two NSMs should have - the same API end-points.)

And we can signal support (at least for NewSM) by setting the X-NSM-Capable and X-NSM-Exec keys inside LiSP's .desktop file (jackaudio/new-session-manager#40).

[moonshaadow]

+1 for NSM support. Currently working with a hack in RaySession, but a full NSM support would be nice to let the session manager control the ability of save the show and show/hide the GUI.