Make the label prop reactive

[valgeirb]

I ran into a problem when using the label prop dynamically and with a custom element. My particular problem was that when using Vue I18n:

<VueCtkDateTimePicker :label="$t('components.dateTimePicker.label')">
  <FilterButton />
</VueCtkDateTimePicker>

The text would just become "COMPONENTS.DATETIMEPICKER.LABEL" instead of changing to the corresponding translation.