Show full relative path for reference source text files

Instead of showing just the base name (e.g. "[messages.json]"), use
relative path (e.g. "[../en/messages.json]").

This makes a difference in the not-uncommon setup where the filename is
the same, but located in an adjacent folder.

src/edframe.cpp

@@ -2497,18 +2497,23 @@ void PoeditFrame::OfferSideloadingSourceText()
     if (!Language::TryGuessFromFilename(filename, &wildcard).IsValid())
         return;
 
+    wxFileName fn(filename);
+
     wildcard.Replace("*", "en");
     wxFileName ref(wildcard);
-    if (!ref.FileExists() || wxFileName(filename) == ref)
+    if (!ref.FileExists() || fn == ref)
         return;
 
+    wxFileName displayRef(ref);
+    displayRef.MakeRelativeTo(fn.GetPath());
+
     AttentionMessage msg
         (
             "sideload-symbolic-id-source",
             AttentionMessage::Question,
             _("Would you like to use English for source text?")
         );
-    msg.SetExplanation(wxString::Format(_(L"This file uses string IDs instead of source text. Poedit can load English texts from the “%s” file for you."), ref.GetFullName()));
+    msg.SetExplanation(wxString::Format(_(L"This file uses string IDs instead of source text. Poedit can load English texts from the “%s” file for you."), displayRef.GetFullPath()));
     // TRANSLATORS: Shown as action button when asking if the user wants to replace string IDs with English text; "load" as in "load from file"
     msg.AddAction(_("Load English"),[=]{
         SideloadSourceTextFromFile(ref);

src/edlistctrl.cpp

@@ -717,7 +717,8 @@ void PoeditListCtrl::UpdateColumns()
     if (m_catalog->HasSideloadedReferenceFile())
     {
         wxFileName fn(m_catalog->GetSideloadedSourceData()->reference_file->GetFileName());
-        sourceTitle += wxString::Format(" [%s]", fn.GetFullName());
+        fn.MakeRelativeTo(wxFileName(m_catalog->GetFileName()).GetPath());
+        sourceTitle += wxString::Format(" [%s]", fn.GetFullPath());
     }
     m_colSource->SetTitle(sourceTitle);