Feature Request: Blur transparent borders

[Oneechan69]

(copy and pasted from #2294 by @MonstrousOgre)

I've made the border colors match the terminal/qt background, including the transparency, but the window is actually blurred even though the border isn't. This looks odd. I'd love for the borders to be blurred too.

Exaggerated border size to show what I mean:

[Axenide]

This would be great. I'm working on some themes that could really use this setting.

[vaxerski]

#10491