Add WatermarkForeground property to TextBox, AutoCompleteBox, CalendarDatePicker, NumericUpDown

[jsuarezruiz]

What does the pull request do?

Adds WatermarkForeground property to customize the watermark/placeholder text color for TextBox, AutoCompleteBox, CalendarDatePicker and NumericUpDown controls.

NOTE: This follows the same pattern as the existing ComboBox.PlaceholderForeground property.

What is the current behavior?

Currently, there is no way to customize the watermark text color directly, requires applying changes in styles.

What is the updated/expected behavior with this PR?

Now can set a custom foreground color for the watermark text:

<TextBox Watermark="Search..." WatermarkForeground="Red"/>
<NumericUpDown Watermark="Amount" WatermarkForeground="Orange"/>
<AutoCompleteBox Watermark="Type..." WatermarkForeground="Green"/>
<CalendarDatePicker Watermark="Date" WatermarkForeground="Purple"/>

When WatermarkForeground is set, the custom color persists across all visual states (normal, hover, focus, disabled), matching the behavior of ComboBox.PlaceholderForeground.

How was the solution implemented (if it's not obvious)?

• TextBox is the primary owner of WatermarkForegroundProperty
• Other controls (AutoCompleteBox, CalendarDatePicker, NumericUpDown) use AddOwner pattern
• Fluent theme: Default setter uses TextControlPlaceholderForeground resource
• Simple theme: Default setter uses ThemeForegroundLowBrush resource
• Template uses TemplateBinding WatermarkForeground for the watermark TextBlocks

Checklist

• Added unit tests (if possible)? / Also added render tests
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #20173

[jsuarezruiz]

Should cover also #20302 in the same PR? or better split in two different steps/versions (v11, v12)?

[cla-avalonia]

@jsuarezruiz,

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[MrJul]

Remove this #else, we don't support Direct2D anymore.

[robloo]

We really should have standardized on PlaceholderText terminology I think. It's much clearer and aligns with WinUI... also the ComboBox.

[MrJul]

Note: due to the change of behavior in text boxes (the watermark color doesn't change anymore depending on the state), I've removed the backport label.

Thus, I think we can go and rename Watermark to Placeholder in this PR (keep Watermark on TextBox only and make it obsolete, redirecting the property and get/set methods to Placeholder).