Allow specifying default date/time for "inp_edit_date_time" control

Plan:

1. the new "picker_default" field option would be implemented, that can have 2 type values:

• exact timestamp
• relative timestamp in any format supported by "strtotime" function (e.g. "midnight +1 day -1 second" would resolve to "23:59:59" of today)

1. the new "picker" format would be added to "kDateFormatter" class, that would work like this:

• if field does have value take it
• if field doesn't have any value AND "picker_default" is specified, then use above rules to create desired timestamp
• format timestamp from above using default format of the field (as if empty "format" was specified instead of "picker")

1. in the "inp_edit_date_time" control use "picker" format (instead of no format) for the hidden field, that name starts with "full_date_"