Do not recreate table schema if table did not change

When the table schema was not modified in the Edit Table dialog, do not
do anything when clicking the OK button.

See issue #1686.

src/sqlitedb.cpp

@@ -1425,6 +1425,10 @@ bool DBBrowserDB::alterTable(const sqlb::ObjectIdentifier& tablename, const sqlb
         }
     }
 
+    // Check if any changes were made to the table schema
+    if(old_table == new_table)
+        return true;
+
     // Create savepoint to be able to go back to it in case of any error
     QString savepointName = generateSavepointName("renamecolumn");
     if(!setSavepoint(savepointName))