DGNIMPORT: reference naming, cells, text placement, curves, references

Name clashes, DGNIMPORTMODE and prefixing as the reference names them;
V7 cell copies, unshared cells and mirrored placements as blocks;
justified text and text nodes placed at their justification points with
the reference's MTEXT contents; B-spline curves as splines and 3D arcs and
ellipses in their own planes; every V8 level as a layer; design models
only; reference attachments translated, ignored or attached as underlays.
The colour index palette comes from the codec fix it depends on.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

locales/ru-RU/opencadstudio.ftl

@@ -6673,7 +6673,10 @@ dgn-import =
     .references = External DGN references
     .bind = Bind as block / xref
     .attach = Attach as DGN underlay
-    .references-note = Reference attachments of DGN files are not read; they are left out.
+    .existing = When the translated drawing exists
+    .prompt-overwrite = Prompt to overwrite
+    .overwrite = Overwrite without prompting
+    .keep-existing = Do not overwrite
     .mapping = Property mapping
     .mapping-standard = Standard mapping setup.
     .mapping-note = The preview shows the selected setup's effect on this file.