Recognize Shopify-style en.default.json for sideloading

Offer to load en.default.json (as well as default.json or default.po
while we're at it) when opening files with symbolic IDs for source text,
in addition to previously used en.json.

This naming convention is used by Shopify, see
https://shopify.dev/docs/storefronts/themes/architecture/locales/storefront-locale-files

Author: vslavik

src/edframe.cpp

@@ -2419,10 +2419,20 @@ void PoeditFrame::OfferSideloadingSourceText()
         return;
 
     wxFileName fn(filename);
+    wxFileName ref;
 
-    wildcard.Replace("*", "en");
-    wxFileName ref(wildcard);
-    if (!ref.FileExists() || fn == ref)
+    for (auto candidate: {"en", "en.default", "default"})
+    {
+        auto w(wildcard);
+        w.Replace("*", candidate);
+        wxFileName fnw(w);
+        if (fnw.FileExists() && fnw != fn)
+        {
+            ref = fnw;
+            break;
+        }
+    }
+    if (!ref.IsOk())
         return;
 
     wxFileName displayRef(ref);