[Proposal]: Allow NavigationView icons beyond PathGeometry (SvgImageSource / Element)

[asural]

Problem statement

NavigationView.Items currently only accepts icons as Func<T, PathGeometry?>.
Apps that want richer SVG icons (especially via Aprillz.MewUI.Svg / SvgImageSource)
cannot use them directly in the navigation pane.

Today the practical workaround is to extract SVG path d strings and call
PathGeometry.Parse. That works for simple monochrome icons, but it does not
compose well with multi-path SVGs, tinting, or the SvgImageSource workflow
already documented for Image controls.

Why the current approach is insufficient

1. PathGeometry-only icons force a second icon pipeline alongside MewUI.Svg.
2. SvgImageSource + Image works elsewhere, but NavigationView cannot consume it.
3. Converting every icon to a single path string loses SVG flexibility
   (multiple fills, strokes, complex groups) and is hard to maintain for large
   Fluent / custom icon packs.
4. App-level workarounds (custom sidebar, drawing icons manually) abandon
   NavigationView's built-in icon slot and rail/compact layout behavior.

Type of change

Public API change

Expected impact

• App developers using NavigationView + SVG icon packs can keep one icon source.
• Existing PathGeometry-based apps remain unaffected if the API is additive
  (overload / optional alternate selector).
• Slightly broader icon rendering path in NavigationView pane items.

Alternatives or tradeoffs

Alternatives already available in app code:

• Keep using PathGeometry.Parse with Fluent SVG d paths (current workaround).
• Build a custom navigation list instead of NavigationView.
• Rasterize SVGs to bitmaps (worse for DPI / theme tint).

Tradeoffs if added to MewUI:

• Need a clear icon abstraction (PathGeometry vs IImageSource vs Element).
• Pane measurement/layout must handle non-path icons consistently in
  expanded / compact / rail modes.
• Prefer additive API to avoid breaking existing PathGeometry callers.

Additional context

Current signature (0.19.1):

.Items(
items,
text: item => item.Text,
icon: item => item.Icon, // PathGeometry only
kind: item => item.Kind,
keySelector: item => item.Key,
content: ...)

Possible directions (any would help):

1. Overload icon selector to accept IImageSource / SvgImageSource
2. Allow Func<T, Element?> for custom icon content
   (e.g. Image { Source = SvgImageSource.From... })
3. If PathGeometry-only is intentional, document the recommended SVG → PathGeometry
   workflow for NavigationView

Related package: Aprillz.MewUI.Svg (SvgImageSource + Image already works)

Environment:

• Aprillz.MewUI 0.19.1
• Considering Aprillz.MewUI.Svg for navigation icons

Before submitting

• I searched for existing issues and discussions first.
• I described why this belongs in MewUI core rather than only in app-level code.

[al6uiz]

Thanks for the proposal!

The initial version of NavigationView intentionally limited icons to PathGeometry, rendered through a PathShape, to keep the navigation items visually consistent in size, alignment, and theme behavior.

However, supporting richer icon content is a reasonable extension. I am considering an API that accepts the common UI base type, Element, rather than introducing an image-specific abstraction. This would allow applications to use Image, including SvgImageSource, a TextBlock for Emoji, or as well as other custom elements while retaining control over their presentation.

The API also needs to make sizing, alignment, compact-mode behavior, and theming sufficiently predictable and easy to customize. I will review the appropriate API shape with those constraints in mind.

This will most likely be included in the next major release, v0.20.0.