Position plugin popups inside the capture area

[apohl79]

Hi,

I'm making another suggestion related to the display. It would be nice to choose also the frame anchor to have popup windows also visible. With Kontakt, for instance, I have to go fullscreen in order to see the popup, which is time-consuming and can sometimes be confusing as it makes Kontakt look frozen.

Originally posted by @Manaberry in https://github.com/apohl79/audiogridder/discussions/1178#discussioncomment-5379397