There is no such option in Keyboard Layout Options which is sad.
How can I do it manually?
To do this manually you want to create a file called .Xmodmap in your home directory. You could do this using a command from the terminal
That will create the file and open it in gedit. Add the following lines to the file:
Save the file and quit gedit. Next time you login the new keymappings will be active. To have the settings take immediate effect run the following command:
There's a chance that the keycodes above are different on your computer. If the above doesn't work, try running the xev program from the command line. Xev will generate a new small window in the corner of your screen. If you start pressing keys on your keyboard you’ll get all the key mapping information for that key press.
For example if I press “q” when xev is running I get the following output:
keycode 24 is the information that we're after. Press your left Ctrl and left Alt buttons while running xev to get your own respective keycodes to confirm that they are the same as the ones used above.
Hope that helps.