Add an "active" property to GimpFilter, which replaces its
"visible" property.  The new property assumes the lower-level role
"visible" had -- controlling whether the filter has any effect as
part of its parent filter-stack.

Add a "visible" property to GimpItem, separate from the "active"
property, which assumes the higher-level role "visible" had --
controlling whether the item is considered "visible", as per the
GUI.  By default, the item's "visible" property is bound to the
filter's "active" property, so that changes in visibility directly
affect the filter's "activeness"; this binding can be controlled
using the new gimp_item_bind_visible_to_active() function.

This distinction is currently necessary for floating selections.
Floating selection layers must not be active in their parent stack,
regardless of their visibility, in particular, so that their mode
node doesn't hide the entire backdrop when their composite mode
excludes the backdrop (i.e., when it's dst-atop or src-in).
Instead, their visibility should affect the activeness of the
floating-selection filter of the drawable they're attached to.
This is handled by the next commit.