Textfield (Guidelines 3.0)

[mfranzke]

The following changes will be part of the Guidelines 3.0 to the textfield element:

• add variants
• add colors
• add different states
• remove unused styles

Additionally

• CHANGELOG.md entries, both for prereleases (underneath Unreleased at the beginning) as well as in a hidden (by comment) summary at the beginning
• docs/migrationGuide.adoc
• Test-URLs: .pa11yci and tests/backstop.json