import: IMPORT window and DGNIMPORT converting DGN models to drawing objects

The DGN V7/V8 readers expose each element (level, symbology, geometry,
complex chains, text nodes, shared cells). DGNIMPORT / -DGNIMPORT convert a
model with the Standard mapping into a new drawing; IMPORT opens the
Import File window and routes PDF and DGN files to their importers.

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

src/ui/window/mod.rs

@@ -15,6 +15,7 @@ pub mod drawing_units;
 pub mod field_dialog;
 pub mod find_replace;
 pub mod geometric_tolerance;
+pub mod import_dialogs;
 pub mod gradient_editor;
 pub mod graphic_attributes;
 pub mod layer_state_manager;