Add option for custom date time format

Found another bug where it would still validate when leaving the end-date empty.

ps. an interdiff doesn't seem to generate : interdiff: Whitespace damage detected in input

Found another small issue where it would strip the time from the entered values. And the end-value wasn't handled correctly.

Here's a new patch.

The new option to configure a custom format per field widget works as expected, thanks!

@idebr, Thanks for the review. Why did you hide the patch from #23 and #16. Are you saying the one you tested is the last one not hidden (#14) ?

Please clarify and hide all files other than the one you tested, please..

I was unaware I changed the displayed patches. I tested the latest patch in #23.

Attached patch implements the following changes:

1. Applied default value formatting to single_date_time_timestamp_widget and single_date_time_widget
2. Applied massageFormValues() to all widgets that extend SingleDateTimeBase

I tested patch #28, looks good the default_value formatting is now correct.

Rerolled the latest patch for 2.0.0-beta2.

I can confirm that this works correctly. Thanks for the patch.

Unfortunately, patch number 30 doesn't work for me. There is one small mistake which causes on each save to delete the date from the fields. I have changed $element['value']['#custom_format'] ?? $format to $element['value']['#custom_format'] ?: $format as the #custom_format is actually empty string by default.