IMPORT: 3D Studio files

A .3ds file opens the 3D Studio Import window: the file's objects with
their kind and material count, filtered by kind or name, checked one by
one or all at once, where to put them on layers and what a mesh with
several materials becomes. Checked meshes come in as polyface meshes on
the chosen layers with materials made from the file's colours, cameras
as named views; a mesh with several materials is split, given its first
material, given none, or asked about on the command line. Automation
drives the window with tds_import_dialog.

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

src/app/mod.rs

@@ -2335,6 +2335,8 @@ pub enum ModalKind {
     ImportFile,
     /// DGNIMPORT's DGN Import window.
     DgnImport,
+    /// IMPORT's 3D Studio Import window.
+    TdsImport,
     /// Web: a crash-recovery autosave snapshot newer than the drawing's last
     /// save awaits a restore/discard decision. Distinct from the corrupt-file
     /// `RecoveryPrompt` — nothing is wrong with the drawing itself.