i18n: Add support for translating the class reference

- Parse `.po` files from `doc/translations/*.po` like already done
  with `editor/translations/*.po`.
- Add logic to register a doc translation mapping in `TranslationServer`
  and `EditorSettings`.
- Add `DTR()` to lookup the doc translation mapping (similar to `TTR()`).
  Strings are automatically dedented and stripped of whitespace to ensure
  that they would match the translation catalog.
- Use `DTR()` to translate relevant strings in `EditorHelp`,
  `EditorInspector`, `CreateDialog`, `ConnectionsDialog`.
- Small simplification to `TranslationLoaderPO`, the path argument was
  not really meaningful.

editor/SCsub

@@ -66,10 +66,15 @@ if env['tools']:
 
     path = env.Dir('.').abspath
 
-    # Translations
+    # Editor translations
     tlist = glob.glob(path + "/translations/*.po")
-    env.Depends('#editor/translations.gen.h', tlist)
-    env.CommandNoCache('#editor/translations.gen.h', tlist, run_in_subprocess(editor_builders.make_translations_header))
+    env.Depends('#editor/editor_translations.gen.h', tlist)
+    env.CommandNoCache('#editor/editor_translations.gen.h', tlist, run_in_subprocess(editor_builders.make_editor_translations_header))
+
+    # Documentation translations
+    tlist = glob.glob(env.Dir("#doc").abspath + "/translations/*.po")
+    env.Depends('#editor/doc_translations.gen.h', tlist)
+    env.CommandNoCache('#editor/doc_translations.gen.h', tlist, run_in_subprocess(editor_builders.make_doc_translations_header))
 
     # Fonts
     flist = glob.glob(path + "/../thirdparty/fonts/*.ttf")