Change argument of get_layer and get_mdi_zorder function pointers to be

Change argument of get_layer and get_mdi_zorder function pointers to be

* atk/atkcomponent.h:
Change argument of get_layer and get_mdi_zorder function pointers
to be AtkComponent instead of AtkObject

* atk/atkcomponent.c:
Change atk_component_get_layer and atk_component_get_mdi_zorder
to refer to argument as component instead os accessible

* docs/atk-sections.txt docs/tmpl/atkaction.sgml
docs/tmpl/atkcomponent.sgml docs/tmpl/atkeditabletext.sgml
docs/tmpl/atkimage.sgml docs/tmpl/atkobject.sgml
docs/tmpl/atkselection.sgml docs/tmpl/atkstreamablecontent.sgml
docs/tmpl/atktable.sgml docs/tmpl/atktext.sgml docs/tmpl/atkvalue.sgml:
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