Speed up finding message catalog by name in wxTranslations.

Use a helper hash map to allow efficiently finding message catalogs by name
instead of using linear search in a linked list which can be noticeably slow
when many catalogs are used.

This speeds up wxGetTranslation() when the domain is specified.

Closes #16975.

include/wx/translation.h

@@ -183,6 +183,12 @@ private:
     wxTranslationsLoader *m_loader;
 
     wxMsgCatalog *m_pMsgCat; // pointer to linked list of catalogs
+
+    // In addition to keeping all the catalogs in the linked list, we also
+    // store them in a hash map indexed by the domain name to allow finding
+    // them by name efficiently.
+    WX_DECLARE_HASH_MAP(wxString, wxMsgCatalog *, wxStringHash, wxStringEqual, wxMsgCatalogMap);
+    wxMsgCatalogMap m_catalogMap;
 };