Get rid of that pesky forceful dark mode intrusion

Since 6.5.0, there is new infamous dark mode detection which automatically switches to dark palette when system "dark mode" is enabled, resulting in dark "Fusion" (and some others) style.

I do want to have some option to DISABLE this new behavior which actually breaks some older applications!

The culprit seems to be this piece of code from

https://github.com/qt/qtbase/blob/dev/src/gui/kernel/qplatformtheme.cpp#L358

Now, all old application newly compiled with Qt 6.5.0 switch to dark mode (if system is dark mode), which is not always desired.

I want to have some easy to set property (for example via "QStyleHintsPrivate::setColorScheme).

Also, add a property to "QStyle" (and subclasses) which says if the particular style actually supports "dark mode" or not as again - this is now hardcoded in styles and the information is impossible to have in apps. It was already there but was reverted out.

https://github.com/qt/qtbase/commit/c62c2aac752c6b63d5afd2f7a10dd9f43c99ae7b