Add MenuConfig::submenu_gap to override the hardcoded gap between a menu and its submenus

[kiwirm]

The gap between a menu and the submenu it opens is hardcoded in SubMenu::show
(frame.total_margin().sum().x / 2.0 + 2.0), so a submenu can't be placed flush
against its parent — even with a zero menu margin a ~2px gap remains.

This adds an optional submenu_gap: Option<f32> to MenuConfig (plus a
MenuConfig::submenu_gap(f32) builder). When set it overrides the offset; when
None it falls back to the existing computed value, so existing menus are
unaffected. Because a menu's MenuConfig propagates to its submenus, setting it
once on the root menu applies to the whole tree.

egui::MenuButton::new("Add")
    .config(egui::MenuConfig::new().submenu_gap(0.0)) // flush submenus
    .ui(ui, |ui| { /* … */ });

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[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/8338-submenu-gap

View snapshot changes at kitdiff

[lucasmerlin]

Should this be

Suggested change

	let gap = menu_config
	.submenu_gap
	.unwrap_or_else(|| frame.total_margin().sum().x / 2.0 + 2.0);
	let gap = frame.total_margin().sum().x / 2.0 + menu_config
	.submenu_gap
	.unwrap_or(2.0);

instead?
frame.total_margin().sum().x / 2.0 should basically mean "0 visual gap between the menus" which I think is what you'd expect when setting the gap to 0? Otherwise at 0 there would be some overlap.