Expose customizable delegate or titleComponent for submenu titles in Qt Quick Controls 2 Menu

Currently, the Menu component in Qt Quick Controls 2 allows setting a plain string title for submenu entries and optionally an icon.source property. However, the submenu title delegate (the visual item representing the submenu inside its parent menu) is not customizable: margins, padding, spacing between icon and text, or replacing the title with a custom layout are not supported.

This limitation leads to inconsistent UI experiences across platforms and styles and forces developers to implement fragile and complex workarounds — such as replacing the submenu title with custom Items and manually handling submenu positioning and open/close logic.

Proposed Enhancement:

• Introduce a titleComponent or titleDelegate property on Menu to allow fully customizable submenu title content.

• Alternatively, expose the internal submenu title titleItem for style and layout customization.

• Ensure that properties like padding, margin, or spacing between icon and text can be adjusted for submenu titles.

• Maintain consistent behavior with icon.source for both top-level menus and submenu entries.

Use Case:
A typical use case is a submenu titled "Settings" with a gear icon to the left and a right-pointing arrow to indicate a submenu. Developers want to adjust left margin and spacing to align with app style guides. This is currently impossible without custom hacks, which complicate maintenance and reduce reliability.

Benefits:

• Cleaner, more maintainable code for complex menus.

• Consistent styling and behavior across platforms and styles.

• Enables richer, more user-friendly menu UIs with proper icon/text layouts.