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.

(cherry picked from commit 4857648a16)

editor/connections_dialog.cpp

@@ -1001,7 +1001,7 @@ void ConnectionsDock::update_tree() {
 					while (F && descr == String()) {
 						for (int i = 0; i < F->get().signals.size(); i++) {
 							if (F->get().signals[i].name == signal_name.operator String()) {
-								descr = F->get().signals[i].description.strip_edges();
+								descr = DTR(F->get().signals[i].description.strip_edges());
 								break;
 							}
 						}