the gtk a11y rather assumes that the a11y things "belong" to the matching
widget/component and those owners will dispose them, so follow that pattern
here and dispose the a11y objects when the parent is disposed

to reproduce, enable a11y, load a csv, click once in the first column
of the preview area and cancel the dialog and close libreoffice

Change-Id: Ib830da499e9f2d6fed94fb12ede7c929b607ab10
Reviewed-on: https://gerrit.libreoffice.org/29301Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>