Add layer-mask support for group layers.  Group-layer masks work
similarly to ordinary-layer masks, with the following
considerations:

The group's mask size is the same as group's size (i.e., the
bounding box of its children) at all times.  When the group's size
changes, the mask is cropped to the new size -- areas of the mask
that fall outside of the new bounds are discarded and their data is
lost (sans undo), and newly added areas are filled with black (and
hence are transparent by default).

The new gimp_group_layer_{suspend,resume}_mask() functions can be
used to modify this behavior.  Between the outermost pair of
suspend/resume calls, the old mask data is remembered, and is used
to fill the newly added areas while cropping the mask when the
group is resized.  We override GimpItem::{start,end}_move() for
GimpLayer, to call these functions (suspend() in start_move(), and
resume() in end_move()) for each of the layer's ancestors.

As a result, while moving a layer, or a set of layers, atomically,
such as while dragging with the move tool, or moving linked layers,
the ancestors' mask data is not lost, and is only discarded at the
end of the operation.

This commit also takes care of properly handling undo for group-
layer mask crops, properly invalidating the image when the group
layer's mask is shown, and enabling the mask actions for group
layers (obviously :).