Trigger value save on year/month select

[skjnldsv]

See #137 (comment)
Closes #137

@nextcloud/vue

[skjnldsv]

🎺

[ChristophWurst]

👍

[skjnldsv]

@caugner this actually caused an issue: nextcloud/contacts#1196

Is this needed when you don't use confirm?
In contacts we do use confirm and this pull request actually introduced the bahaviour you described here: mengxiong10/vue2-datepicker#290 (comment)

Now in contacts if you click the year in the header, then select a year, then a month and another date, the year is back to what it previously were.

[caugner]

@skjnldsv I have passed the question on to the author of vue2-datepicker: mengxiong10/vue2-datepicker#290 (comment)

[caugner]

@skjnldsv The vue2-datepicker has provided the following fix:

    handleSelectYear(year) {
      let value = this.$refs.datepicker.currentValue;
      if (value) {
        value = new Date(new Date(value).setFullYear(year));
        this.$refs.datepicker.selectDate(value);
      }
    },
    handleSelectMonth(month) {
      let value = this.$refs.datepicker.currentValue;
      if (value) {
        value = new Date(new Date(value).setMonth(month));
        this.$refs.datepicker.selectDate(value);
      }
    }