iconGrid: Add minRows/minColumns properties

When we adapt the grid to different display sizes,
we don't want the number of displayed items to get
too small. In the future we will scale down icons to
make sure that the grid fits add least minRows
x minColumns items, but for now we only take the
properties into account when calculating the dynamic spacing.

https://bugzilla.gnome.org/show_bug.cgi?id=706081