Empty the _id and _prefroot cache after migrating stored preferences.
Empty the _id and _prefroot cache after migrating stored preferences.

Fixes #1157.

Martii/greasemonkey,the8472/greasemonkey,HasClass0/greasemonkey,Ventero/greasemonkey,HasClass0/greasemonkey,the8472/greasemonkey,Ventero/greasemonkey,Martii/greasemonkey