Use GimpObjectQueue, added in the previous commit, in various
instances where we perform an action on a set of objects.  This
improves progress reporting, by using a single progress for the
entire operation, rather than reporting the progress of each object
individually, and by taking the relative cost of each object into
account, instead of assuming a uniform cost for all objects.

In particular, this affects the various whole-image operations
(i.e., transformations and color conversions), operations on linked
items, and operations on layer groups.  This also affects layers
with masks, whose progress is now reported together instead of
individually.

Additionally, this commit fixes erroneous group-layer mask cropping
during undo when resizing the image, by properly calling
{start,end}_move() on all the resized layers before starting the
operation, and when scaling the image, by only scaling top-level
layers, and letting group layers scale their children themselves.