Sync nightcolor plasmoid inhibited state with kwin

Hello! This is my first commit to KDE software, first, thank you all for providing great codes! Now finally I (hope I can) start to make some contributions 😀

The commit fixes the following bug in the night color control plasmoid: If I set a keyboard shortcut for "Toggle Night Color" in system settings - shortcuts - system services: kwin - toggle night color, and if I add a night color control plasmoid in my desktop panel, then the keyboard shortcut and the mouse click on the plasmoid (to switch on/off night color) cannot cooperate together, i.e. I cannot switch on night color by keyboard and switch off by mouse click, or vice versa.

The bug is caused by the inconsistency between the internal states of the plasmoid and kwin. So I add some codes to sync the state from kwin to plasmoid.

Note the commit need another commit for kwin to fully work. kwin!1122 (closed)

I am a beginner here so maybe I made some stupid mistakes, I appreciate it if anyone could point out any other clever way to fix the bug, thanks!