... raises a CRITICAL

gimp_item_{start,end}_move() currently serves two different
purposes:  It is used by GimpLayer to suspend/resume mask resizing
of the layer's ancestors; this is necessary whenever an operation
on a layer might affect the size of its ancestors.  It is also used
by GimpGroupLayer to suspend/resume its own mask resizing; this, on
the other hand, is only necessary before applying one of the
transformation functions to the group, so that mask modification is
handled by GimpLayer.  In other words, the effects of
gimp_item_{start,end}_move() on group layers are only necessary in
a subset of the cases in which these functions are used.

While in itself this isn't a problem, it does cause issues when
removing a group layer:  gimp_image_remove_layer() calls
gimp_item_start_move() before removing the layer, and
gimp_item_end_move() afterwards.  While the former function is
called while the layer is still attached to the image, the latter
function is called after the layer is no longer attached.  Since
GimpGroupLayer pushes an undo step in response to these calls, only
the call to start_move() results in an undo step, while the call to
end_move() doesn't, resulting in an unbalanced
GIMP_UNDO_GROUP_LAYER_START_MOVE undo step on the stack.  This
causes problems when undoing the operation.

Add gimp_item_{start,end}_transform() functions, and corresponding
GimpItem::{start,end}_transform() virtual functions, which are more
specialized versions of gimp_item_{start,end}_move(), which should
be used instead of the former before/after transforming an item; in
other cases, such as when removing ot reordering an item,
gimp_item_{start,end}_move() should still be used.  The default
implementation of GimpItem::{start,end}_transform() calls
gimp_item_{start,end}_move(), respectively, so subclasses that
override these functions don't have to do that themselves.

In GimpGroupLayer, override GimpItem::{start,end}_transform(),
instead of GimpItem::{start,end}_move(), for the same purpose of
suspending mask resize.  This avoids these functions from being
called when removing a layer group, fixing the bug.