greeter mode: Sync screen reader settings to org.mate.applications-at-visual if launching a MATE session.

[sunweaver]

<schemalist gettext-domain="mate-desktop">
  <schema id="org.mate.applications-at-visual" path="/org/mate/desktop/applications/at/visual/">
    <key name="exec" type="s">
      <default>'orca'</default>
      <summary>Preferred Visual assistive technology application</summary>
      <description>Preferred Visual assistive technology application to be used for login, menu, or command line.</description>
    </key>
    <key name="startup" type="b">
      <default>false</default>
      <summary>Start preferred Visual assistive technology application</summary>
      <description>MATE to start preferred Visual assistive technology application during login.</description>
    </key>
  </schema>
</schemalist>

@sunweaver: I think we can close this. We've achieved the desired result via #10 and #12.