applets/brightness: Add Dark Mode toggle
Now that we have an option for which look and feel package is light and which is dark we can finally add a simple toggle that switches between the two.
I have taken the liberty of making the Settings button less intrusive (will do that for Night light, too, if we want this). It opens the landing page right now rather than lookandfeel kcm because the former is where you can always decide light vs dark theme independent of the automatic setting
It also updates live when I change the setting from System Settings landing page.
