"Control caching" modal doesn't mark the pane as changed

Enclosed patch fixes this by adding a data.type 'dismiss-changed'.
Has been tested on both panels_page and panels_mini. Works smoothly.

Note that I couldn't do anything about the display-wide cache settings, as we simply don't have anything that can be marked as changed. This can be a separate issue though, as we'd also need to mark the display changed if a pane has been removed, which currently doesn't give a 'changed' hint either.

Please review and tell me if the way I accomplished it is the one you'd like to choose.