Add option to show sortable columns hint icon

[w-ahmad]

Description

Adds an opt-in TableView.ShowSortableColumnIcon property that shows a subtle hint icon on sortable columns that have no active sort direction, so users can tell at a glance which columns support sorting without having to click them first. The property defaults to false, so no existing app changes visually unless it opts in.

• New TableView.ShowSortableColumnIcon (bool, default false).
• New Sortable visual state on TableViewColumnHeader's SortStates group — shows the header's sort icon at 50% opacity with a new glyph when a column is sortable but currently unsorted.
• Respects both TableView.CanSortColumns and the column's own CanSort — never shows on a non-sortable column.
• Added change-callbacks for CanSort and CanSortColumns (previously missing, unlike the equivalent CanFilter/CanFilterColumns callbacks), so the header now also reacts live if those change at runtime.
• Sample: new "Show Sortable Column Icon" toggle on the Sorting page.
• Docs: new "Showing a hint icon for sortable columns" section in sorting.md, plus an entry in the options table.

Related Issue

Closes #114

Type of Change

• ✨ New feature
• 📝 Documentation update
• 🧪 Test

Checklist

• This PR is not from my main branch
• Tested with WinUI target
• Tested with Uno Platform target
• Unit / integration tests added or updated
• Documentation updated to reflect changes
• Code follows the project's coding conventions

Screenshots / Recordings

Additional Notes

Uno target builds cleanly but wasn't run/visually verified. Worth a quick look in Visual Studio at the new glyph (\ue8cb) next to the existing ascending/descending icons before merging, to confirm it reads well at 10px.

[Copilot]

🟡 Changes recommended

Initially enabling the property before header creation leaves the hint icon hidden.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds an opt-in hint icon for sortable, unsorted columns.

Changes:

• Adds ShowSortableColumnIcon and runtime update callbacks.
• Adds the sortable visual state, glyph, tests, sample, and documentation.

File summaries

File	Description
tests/TableViewSortableColumnIconTests.cs	Tests icon states and runtime changes.
src/VisualStates.cs	Defines the sortable state.
src/Themes/TableViewColumnHeader.xaml	Renders the hint icon state.
src/Themes/Resources.xaml	Adds the hint glyph resources.
src/TableViewColumnHeader.cs	Selects the sortable visual state.
src/TableView.Properties.cs	Adds the new table property and callbacks.
src/Columns/TableViewColumn.cs	Reacts to CanSort changes.
samples/WinUI.TableView.SampleApp/Pages/SortingPage.xaml	Demonstrates the option.
docs/docs/sorting.md	Documents the feature.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 1
• Review effort level: Balanced

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