Summary:
When detecting the properties of a window, now it shows and lets the user select the window complete class.
If this property is selected in the overlay, `Window class` takes the whole class value, and the option `Match window whole class` is set.

This adds back a feature the old kcm was offering.

BUG: 421542
FIXED-IN: 5.20

Test Plan:
- `Detect window properties` and pick a firefox window
- The property selector shows: `Window class: navigator` and `Whole window class: navigator firefox`
- Selecting the latter set the properties as per summary

{F8334118}

Reviewers: ngraham, #kwin, #plasma, meven

Reviewed By: ngraham, #kwin, #plasma, meven

Subscribers: broulik, davidedmundson, meven, anthonyfieroni, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29791