Add mappings for ins and del #141

The ins and del elements currently are shown as having no accessible object for three of the four platforms. In addition, for ATK it states that exposure should be done via text attribute.

In order to easily support presentation (or exclusion, should the user prefer) of edits marked up via ins and del, I think there should be an accessible object. IAccessible2 has just added roles for each of these: https://github.com/LinuxA11y/IAccessible2/blob/master/api/AccessibleRole.idl#L307. ATK and AT-SPI2 will soon, hopefully: https://gitlab.gnome.org/GNOME/atk/issues/2.

In terms of the mappings for ATK, until the new roles have been created, I think using ROLE_SECTION makes sense (i.e. the same mapping as div because ins and del can have child elements.)

We still need a way to make it easy for ATs in Linux to find these elements. A text attribute (be it formatting or something like diff:del/diff:ins) doesn't make sense to me as Orca doesn't bother getting text attributes unless a user explicitly asks for them via command. Checking all text attributes on the slim chance there's an ins or del will not be performant. Orca does, for a variety of reasons, check object attributes. Thus I think exposure via object attribute is the way to go. Could we use xml-roles for that? If not, an object attribute like diff:del and diff:ins should work.