add autogrid

[icxes]

adds autogrid as suggested in issue 1384

• switches (to/out of) grid view automatically if (over/under) ga_thresh% of files have a file extension that is in ga_ext
• always uses grid view if grid view is also enabled
• configs for enabling and setting ga_thresh and ga_ext

This PR complies with the DCO; https://developercertificate.org/