Use CheckboxBlankCircleOutline if calendar is disabled

[raimund-schluessler]

Is your feature request related to a problem? Please describe.

After #3960 when a calendar is disabled, only the color of the icon-bullet is changed to var(--color-text-lighter). This makes it not obvious that the calendar is disabled, because it's just a different color. If you happen to have calendars with a similar color, or have a color-impaired vision, you are a bit lost.
See here for example:

Describe the solution you'd like

Use a different icon for disabled calendars: CheckboxBlankCircleOutline here

calendar/src/components/AppNavigation/CalendarList/CalendarListItem.vue

Lines 33 to 36 in b9576fc

	<CheckboxBlankCircle
	:title="calendar.enabled ? $t('calendar', 'Disable calendar') : $t('calendar', 'Enable calendar') "
	:size="20"
	:fill-color="calendar.enabled ? calendar.color : 'var(--color-text-lighter)'" />

This would solve the problem:

Describe alternatives you've considered

No response

Additional context

No response

@JuliaKirschenheuter @nimishavijay For the design here.

[nimishavijay]

Good point! Your solution would work great 🚀