Export: Implement exporting and importing of sqlite3 and sql files

Elwig/Windows/QueryWindow.xaml.cs

@@ -38,7 +38,7 @@ namespace Elwig.Windows {
             IList<DbColumn> header;
 
             using (var cnx = await AppDbContext.ConnectAsync()) {
-                var cmd = cnx.CreateCommand();
+                using var cmd = cnx.CreateCommand();
                 cmd.CommandText = sqlQuery;
                 using var reader = await cmd.ExecuteReaderAsync();
                 header = await reader.GetColumnSchemaAsync();