added last selection index

[akevalion]

this pull request address this issue:

• Use last initializationn script when you create a new image #634

[demarey]

Hi @akevalion
Thank you for the PR!
The feature is nice but can be annoying depending the way you use Pharo Launcher.
I would add a check box below the script preview Use this script as default script so that one can choose if we will have a default script selected or not.
A few tests on this feature would also be welcomed!