[WIP] Better admin config form validation

[sreichel]

Description (*)

This is just an idea for better input validations ...

1. a lot of fields have no <validate> node in system.xml
2. no php-side validation for most fields

This PR takes <validation> node to test on php-side.

(supports required-entry, valdidate-digit, validate-number for now)

Note:
<validate-digit> (or others) shoud test against empty input, but this not seem to work (anymore?). So i'd to add required-entry ...

Manual testing scenarios (*)

1. remove validation class from html to bypass js-validation
2. ...

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)
• Add yourself to contributors list

[sreichel]

Not complete, but ready for test.