Audio/Video: Add hint on preferences page pointing out missing dependencies

Like #7626 (closed) only on fresh linux, nightly gajim. USB camera and audio are accessible to Firefox, Skype etc.

Versions

• OS: Ubuntu 19.10 eoan
• Gajim version: 1.1.92+890846fc
• GTK version: 3.24.11
• Python-nbxmpp version: 0.9.92 (nightly)

Steps to reproduce the problem

1. Open Gajim -> Preferences, select Audio/Video tab

Expected behavior

It should be possible to select audio and video input and output devices (and then use them for jingle calls)

Actual behavior

All device selections are greyed out and empty. Voice chat and video chat checkboxes in the "sandwich" menu of the chat windows are greyed out.

(Could it have anything to do with apparmor?)