Unify NightColorManager internal state of inhibited

Hello, this is a commit to fix the bug described here: kdeplasma-addons!74 (closed)

I am a totally beginner here so I might make some stupid mistakes: it seems there are two variables m_inhibitReferenceCount and m_isGloballyInhibited that record the internal state for on/off of night color, doesn't these lead to duplication and potentially inconsistency? I tried to unify them together so now the keyboard shortcut and night color control plasmoid can work together.

Thank you for looking at the PR and hope I could make a small contribution 😀