Custom "trigger" slot for datepicker, datetimepicker, and clockpicker

[6XGate]

Description

Since BDatepicker, BDatetimepicker, and BTimepicker are all implemented using a BDropdown, it would seem logical to add support for a custom trigger slot to these components rather than always being forced to use the BInput control.

Why Buefy need this feature

This would provide further customization, especially in use-cases where the trigger would need to have a different look, but still function as a drop-down as it does now. Having the user re-implementing this with the inline option seems big ask.

An exmple may be a regular solid color button:

<b-datepicker>
  <template #trigger>
    <b-button label="Pick date" type="is-primary"/>
  </template>
</b-datepicker>

[jtommy]

That isn't a bad idea!