Change namespace for extra screenshot functionality

In https://github.com/elementary/gala/pull/633 it looks like we merged the conceal text and area with cursor methods under the org.gnome.shell namespace.

We should change these methods to be under the io.elementary.gala namespace instead.

It might be worth considering that all of these methods should be under io.elementary.gala and we don't need to use org.gnome.shell since applications will be expected to use `org.freedesktop.portal.Screenshot `. This could break compatibility with other screenshot tools I guess, but I think the assumption is that sandboxed apps won't have access to `org.gnome.shell` so it doesn't really matter if we don't provide it