Saving of icon theme in qt6ct.conf is inconsistent with other settings

[pmattern]

If qt6ct is launched for the first time and quit by clicking OK, the majority of settings is saved in ~/.config/qt6ct/qt6ct.conf, but the icon theme isn't. In order to have it saved in qt6ct.conf, its entry in tab "Icon Theme" has to be highlighted by clicking first.
This is inconsistent and can, more importantly, be pretty confusing when launching qt6ct the first time. So I think it would be good, if the icon theme could be saved the same way as the other settings.

Steps to reproduce:

1. Make sure qt6ct isn't running and ~/.config/qt6ct/ is absent.
2. Launch qt6ct and quit it by hitting OK without taking any action.
3. Open qt6ct.conf. The icon theme will be missing in section [Appearance].
4. Launch qt6ct again, highlight an icon theme in tab "Icon Theme" by clicking its entry, quit by clicking OK.
5. Open qt6ct.conf. The icon theme is now present in section [Appearance].

Seen running 55dba87 on Arch Linux.