Add whitelist mode

[saihaze]

With a proper aurorae decoration theme, only a few apps need kwin-enforced rounded corners. For example, Steam, Tencent QQ, and some Gtk applications. Whitelist mode will allow users to enable the effect only for a few windows and reduce GPU usage.

[pviscone]

I was trying to set up this thing. I thought that unchecking "include all normal windows" in the "Inclusions & Exclusions" panel and including just the chosen windows would have worked, but I found a bug.

When you uncheck "include all normal windows", you can't apply the modifications and this will not be saved.
Looking at older issues, there were similar problems [#157].

I think after this fix we will be able to set up a whitelist.

[matinlotfali]

@pviscone thanks for pointing it out. I was missing a plural "s" in widget names 🙈

[pviscone]

Wow, easiest fix ever, thank you!