refactor Wifisecurity backend to compatible QML classes

The new WifiSecuritySetting logic will be almost similar to the original widget code, with one difference.

In the old widget, the four WEP keys lived in the NetworkManager setting object (wepKey0()–wepKey3()).

Now, in the new design, the four keys are cached locally in the setting object as a fixed array:

m_wepKeys

m_wepKeys is now represented as an array of QString which represent all 4 keys:

https://invent.kde.org/plasma/plasma-nm/-/merge_requests/582/diffs?file_path=libs%2Feditorqml%2Fsettings%2Fwifisecurities%2Fwifisecuritysetting.h#line_b45f1074a_A134

It act as single source of truth for all available keys:

https://invent.kde.org/plasma/plasma-nm/-/merge_requests/582/diffs?file_path=libs%2Feditorqml%2Fsettings%2Fwifisecurities%2Fwifisecuritysetting.cpp#line_cd31a39ff_A124

The m_wepKey will show us the active key.