[Feature Request] Support for Plasma's native Background Contrast KWin effect

[ryu-ketsueki]

I don't know why I have neglected this native KWin effect for so long. It not only improves readability on semitransparent panels on the Plasma Desktop, but also allows them to look fancier with custom Plasma Themes.

However, I noticed that Klassy does not make use of that effect, somehow. The contrast settings I enabled on the Plasma Theme only appear for the panels, not the transparent titlebar and toolbar.

Now, I don't know if this is a limitation of how the effect itself work or Klassy's not making use of the effect, but it would be a good idea to look into that.

As an example, here's how the Panel looks without the Background Contrast and a decently configured Plasma Theme

And now with the effect enabled

I don't get that level of contrast, tint or anything of that kind with transparent titlebar and toolbar on Klassy, and it would be a cool addition.