kcms/style: add special case for Adwaita gtk theme
Reason for the change
currently, there's no way for a user to select the default GTK theme, by default they only get breeze, Default and emacs, the latter of which aren't even themes, but key bind settings. this is because the Adwaita theme is embedded in the GTK library, so just scanning for the folder in the themes directory doesn't find it
Test plan
i tested this by building the code in a vm, running ninja install + reboot, and then changing the GTK theme setting between Adwaita and breeze with a gtk3 app open
Screenshots or screen recordings
| Before | After |
|---|---|
| Video_2025-08-08_15-11-36 | Video_2025-08-08_15-05-13 |