Sheet set row menus: own fly-out menu widget

The row menu is a small overlay widget: a click on a submenu entry opens
its fly-out instead of closing the menu, and the fly-out is placed beside
the menu where it actually landed (right when it fits, otherwise left), so
the menu never moves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/window/mod.rs

@@ -27,6 +27,7 @@ pub mod alias_editor;
 pub mod find_replace;
 pub mod named_parameters;
 pub mod pc_manager;
+pub mod row_menu;
 pub mod sheet_set;
 pub mod pdf_dialogs;
 pub mod xref_attach;