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.

src/pavucontrol.desktop.in

@@ -8,3 +8,4 @@ Icon=multimedia-volume-control
 StartupNotify=true
 Type=Application
 Categories=AudioVideo;Audio;Mixer;GTK;
+_Keywords=pavucontrol;Microphone;Volume;Fade;Balance;Headset;Audio;Mixer;Output;Input;