OcTextInput: add defaultValue prop and make clear button null the value

[dschmidt]

Description

1. Fixed the clear button being visible even when element is disabled
2. Made clear button emit null instead of empty string
3. Made input/change event handling a bit more consistent
4. Added defaultValue prop that is for now passed as placeholder to the input field.

Needs UX concept by @kulmann and @tbsbdr

Related Issue

• Fixes one ❌ from Streamline props and events for all form components web#8857 (defaultValue) and makes it more consistent with other components that also emit null on clear (which is not part of that table)

Motivation and Context

Same as #1634 and #1635

Instead of using native placeholder prop on the input element, I would propose to introduce the defaultValue property which for now uses placeholder but keeps that as implementation detail. That way I can already use the functionality in ownBrander 2.0 and will automatically benefit from a better implementation at some point in the future.

How Has This Been Tested?

Screenshots (if appropriate):

Only visual change: it's now possible to have an empty input field with clear button.
That is useful when a user overrides the default value with an empty string.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  we don't emit empty strings anymore on clear, but afaict clearButtonEnabled is used nowhere, so this shouldn't break anything in ownCloud
• Technical debt
• Tests

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation added/updated

Open tasks:

• ...

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

87.5% Coverage
0.0% Duplication

[pascalwengerter]

LGTM and thanks for taking care, would like to get a final approval from @kulmann before merging

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

86.7% Coverage
0.0% Duplication

[kulmann]

LGTM 👍