Enhancement: Support Color, Transparency, and Layout for Watermark

[hq-zhonger]

Description

Currently, the document.Document.AddWatermarkText() function lacks support for configuring:

• Color (e.g., red, blue, etc.)
• Transparency (Opacity) (e.g., 50% opacity)
• Text Layout Options (e.g., rotation, position settings)

For example, a typical watermark should allow customization like:

• Font: Arial Unicode MS
• Text: "CONFIDENTIAL"
• Size: 48
• Color: Red
• Opacity: 50%
• Rotation: 45° diagonal

However, unioffice does not provide built-in APIs for these configurations.

Expected Behavior

Provide API support for:

wm := doc.AddWatermarkText("CONFIDENTIAL")
wm.SetColor(color.Red)      // Set text color
wm.SetOpacity(50)           // Set transparency (50%)
wm.SetRotation(45)          // Rotate text by 45 degrees

[github-actions]

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