Calendar visibility toggle (entry itself) is not focusable via keyboard

[JuliaKirschenheuter]

[JuliaKirschenheuter]

As discussed wit @jancborchardt this issue needs a small improvements:

• Calendar visibility toggle (entry itself) is not focusable via keyboard: and it should not be focusable, because this have a same action as AppNavigationItem
• Calendar visibility toggle should have a better description
• Some improvements were already done in Use CheckboxBlankCircleOutline if calendar is disabled #4005