Fix broken input and display of Date/Time values in Kexi tables and forms

Summary:
The current implementation does not need to compute position of sections and scan for separators, but:

• generate input mask based on data/time/date+time formats
• separate input format from output format
• allow to enter incomplete numbers for values, e.g. for hours even if the format expects two digits, one digit is enough
• capture 'empty format' which helps to detect that only separators are present in the input string
• add fallback from names to numeric values of months and days when generating input mask

Test Plan: Try the test database at https://bugs.kde.org/show_bug.cgi?id=369518#attach_101338

Reviewers: jkunert, piggz

Reviewed By: piggz

Subscribers: Kexi-Devel-list

Tags: KEXI

Maniphest Tasks: T3901

Differential Revision: https://phabricator.kde.org/D2924