Add default extension for native win32 file save dialog.

We manually appended an extension already (so functionality does
not change), however GetSaveFileNameW also adds the chosen name
(which was the name without extension before) to the list of
recently used documents which resulted in unusable links.

(cherry picked from commit 1da36c2b)

src/ui/dialog/filedialogimpl-win32.cpp

@@ -1785,6 +1785,7 @@ void FileSaveDialogImplWin32::GetSaveFileName_thread()
     ofn.lpstrFilter = _filter;
     ofn.nFilterIndex = _filter_index;
     ofn.lpfnHook = GetSaveFileName_hookproc;
+    ofn.lpstrDefExt = L"svg\0";
     ofn.lCustData = (LPARAM)this;
     _result = GetSaveFileNameW(&ofn) != 0;