add com.github.DiegoMMR.CosmicExtAppletNowPlaying

[DiegoMMR]

Adds the Flatpak packaging files for COSMIC Now Playing, a COSMIC applet that displays MPRIS media information in the panel.

I built and tested the Flatpak locally and confirmed the applet runs as expected.

---

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[lorduskordus]

If you want the applet to show up under the COSMIC Store's "Applets" category whenever it gets merged, you should add this to your metainfo:

<provides>
  <id>com.system76.CosmicApplet</id>
</provides>

[DiegoMMR]

@lorduskordus thank you, i just sent the change adding that id