Add keywords to desktop file

In traditional packages, including 'pavucontrol' here is redundant with Exec. However, only the first field of Exec is used as a search key, and in the Flatpak the first field is 'flatpak'. I often search for 'pavucontrol' and fail to find the app as a result.

All the others are terms that seem relevant to finding this app. Most of them are borrowed from gnome-control-center's sound panel's .desktop file.