fix(controls): Add ItemsPanel to ComboBox to enable GroupStyle support

[frostybee]

This PR includes the following:

• Fix ComboBox not rendering grouped items when using GroupStyle (only headers were showing, items were missing)
• Add ItemsPanel property with StackPanel to the ComboBox style to enable proper group rendering
• Add Gallery example demonstrating ComboBox with grouped items

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

When using GroupStyle with a ComboBox, only the group headers would display: the actual items within each group were missing. This was because the ComboBox style was missing an ItemsPanel property that ListBox already had.

Issue Number: N/A

What is the new behavior?

Added the ItemsPanel setter to ComboBox.xaml:

<Setter Property="ItemsPanel">
  <Setter.Value>
	  <ItemsPanelTemplate>
		  <StackPanel />
	  </ItemsPanelTemplate>
  </Setter.Value>
</Setter>

A simple StackPanel is used instead of VirtualizingStackPanel because virtualization can conflict with GroupStyle rendering, and ComboBox dropdowns typically have fewer items where virtualization isn't needed.

Test plan

• Open Gallery app → BasicInput → ComboBox page
• Verify the "grouped items" example shows both group headers AND items
• Verify scrolling works with the scrollbar visible
• Test in both light and dark themes

Other information

[byrdthomas2013-source]

frostybee:main