Change to common definition of "hidden" in ARIA specs

In SVG, it is possible for an element to be invisible (visibility: hidden)
but still interactive to click events (through the pointer-events style
property).  It is used, for example, to create an image-map behavior with a
raster image file, or to create large click regions for small data points
in a chart.

For the SVG accessibility mappings, we therefore want to make sure that
these interactive-but-invisible elements can be included in the
accessibility tree, so that assisstive technologies can forward events
correctly.  The current definition of "hidden" that is shared by all ARIA
specs (and which refers to elements that should be excluded from the
accessibility tree) doesn't factor this in.

Current definition:
Hidden

Indicates that the element
<http://127.0.0.1:55557/svg-aam/svg-aam.html#dfn-element> is not visible or
perceivable <http://127.0.0.1:55557/svg-aam/svg-aam.html#dfn-perceivable>
 to *any* user. An element is considered *hidden* if it or any one of its
ancestor elements is not rendered or explicitly hidden.
Proposed definition:
Hidden

Indicates that the element is not visible, perceivable, or interactive for
*any* user. An element is considered *hidden* if it or any one of its
ancestor elements is not rendered or is explicitly hidden.
The rest of the definition I think is flexible enough to work for both SVG
and HTML.  I've already proposed explicit definitions of "not rendered" in
SVG which is distinct from visibility.

~Amelia