add a global cache for grouped images

When we load the images into the Icon view,
we ask each time, whether there are grouped
images, with 30000 images in the view are that
also 30000 SQL query.
With this patch, the time to load a view with
many images is faster with MySQL 3x and with SQLite 2x.
CCBUGS: 391840
CCBUGS: 398921
CCBUGS: 396086
CCBUGS: 397901