Thin Extrude: walls of a set thickness along sketch curves (#721) (#991)

Thin Extrude builds walls of a set thickness along sketch curves, open chains and closed loops, with the wall centred, inside, outside or on both sides at two thicknesses. Saved under format 20 (thin on an Extrude); an extrude without thin saves as before. Closes #721.

assets/locales/de.json

@@ -702,6 +702,14 @@
   "sketch_text.path_hint": "Eine Skizzenkurve, der der Text folgt: hier klicken, dann die Kurve",
   "sketch_text.path_clear": "Text von der Kurve lösen",
   "sketch_text.on_path": "Der Text folgt seiner Kurve: sie bestimmt seine Lage",
+  "Thin": "Dünnwandig",
+  "Thin Extrude": "Dünnwandig extrudieren",
+  "Wall Location": "Wandlage",
+  "Wall Thickness": "Wandstärke",
+  "Thickness Outside": "Dicke außen",
+  "Inside": "Innen",
+  "Outside": "Außen",
+  "Profiles/Curves": "Profile/Kurven",
   "update.button": "Update",
   "update.button_tip": "SolidCraft {version} ist verfügbar",
   "update.title": "Update verfügbar",