ui5-daterange-picker: allow first and last day to be the same

[MarcusNotheis]

Describe the bug
We have a scenario where we want to offer the user the possibility to create events. We don't know upfront whether the user wants to create a single-day event or a multi-day event. Therefore it would be beneficial if the DateRangerPicker would we able to provide also a one day selection. (e.g. by clicking the same day twice). I know that there is a DatePicker for that use case but in my opinion the user shouldn't decide whether a DatePicker or a DateRangePicker should be render (e.g. via a Multi Day Event switch.)

To reproduce
Steps to reproduce the behavior:

1. Open https://sap.github.io/ui5-webcomponents/playground/components/DateRangePicker/
2. Open the first date range picker and select a date by clicking it twice.
3. The popover now closes and displays the selected value, but onChange isn't fired
4. If you now reopen the DateRangePicker, you are immediately in the selection part of selecting a second date which is confusing to me.

Expected behavior
I would expect that selecting a single date by clicking it twice would select that date as firstDateValue and lastDateValue.

Screenshots

Context

• UI5 Web Components version: 1.0.0-rc.8

Affected components (if known)
ui5-daterange-picker