Adds header rows for grouped views

[eamodio]

Overview

Introduces header rows for grouped views to improve navigation and visual organization when multiple views are displayed together.

Changes

• Adds new GroupedHeaderNode class that creates header entries for grouped views with appropriate labeling and context values
• Enhances view context menu support by adding comprehensive menu contributions for all view types in the view/item/context section
• Improves grouped view presentation by automatically inserting header nodes at the top of grouped view content
• Extends view base functionality with grouped icon support and enhanced description handling for grouped contexts

Benefits

• Better visual hierarchy - Users can easily distinguish between different view sections when multiple views are grouped
• Improved accessibility - Header rows provide clear context about which view section content belongs to
• Enhanced navigation - Context menus are now available directly on view headers for quick access to view-specific actions
• Consistent user experience - All grouped views now follow the same header pattern for uniformity

The implementation automatically detects when views are in grouped mode and adds appropriate header nodes without affecting standalone view behavior.

[d13]

LGTM