Adaptive transparency for panels

We ran into a design issue whereby increasing the transparency of the panel looked bad when a window was maximized against it, impeding our ability to do so. Simultaneously, having an even slightly transparent panel with a window maximized against it results in an inconsistency between the opaque window and the transparent panel. With this patch/change, we get the aesthetic benefits of greater transparency when there isn't a window maximized against the panel, and the functional and consistency benefits of an opaque panel when there is a window maximized against the panel. [~Nate]

adaptiveopacity-2020-08-06_16.17.58.mkv

(Ignore the audio.)

This requires the theme to have the following lines in the metadata desktop file:

[AdaptiveTransparency]
enabled=true

Depends on plasma-workspace!194 (merged) and https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/119

Might also need !92 (merged) to not break margins