Add ability to set window color scheme manually
It's still optional, currently it will attempt to find KF6ConfigWidgets and the related feature will be enabled if KF6ConfigWidgets can be found.
This can be handy for non-Linux users since their OS might not yet offer a native dark color scheme, or they might want to switch to a color scheme which is different from the one the OS currently used.
This patch also set the Qt theme to breeze due to the current Windows11 style have some minor issues. We might also want to add an option to set the application style like what Kdenlive did.
Before the patch (screenshot under Windows 11, build with MSYS2 UCRT64):
After the patch: