plasma-windowed: add debug identifier to debug activeFocusItem
This helps debug a11y in Plasma widgets.
Example command: QT_LOGGING_RULES="org.kde.plasma.sdk.a11y.debug=true" plasmawindowed org.kde.plasma.clipboard
Inspired by https://carlschwan.eu/2023/07/30/debugging-the-keyboard-navigation-in-your-qml-application/
Edited by Fushan Wen