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.

core/io/translation_loader_po.h

@@ -37,7 +37,7 @@
 
 class TranslationLoaderPO : public ResourceFormatLoader {
 public:
-	static RES load_translation(FileAccess *f, Error *r_error, const String &p_path = String());
+	static RES load_translation(FileAccess *f, Error *r_error = NULL);
 	virtual RES load(const String &p_path, const String &p_original_path = "", Error *r_error = NULL, bool p_use_sub_threads = false, float *r_progress = nullptr);
 	virtual void get_recognized_extensions(List<String> *p_extensions) const;
 	virtual bool handles_type(const String &p_type) const;