Allow altering volume of previously seen applications
Some applications only temporarily show up in pavucontrol (TelegramDesktop for example). These volumes might be too loud and not possible to change from within the app itself. There is already a tool for changing volumes of previously seen applications, but it can be useful to have an option inside the playback tab to also show previously seen applications. Sometimes a GUI might be easier for people to work with. It could also give some insight why some applications sound more quiet than others.
I would suggest adding a checkbox for this, as it's applicable to both "Show all streams" and "Show applications"