gtktextviewaccessible: update cursor position after text suppression

delete_range_cb is set to be called before the text suppression done by
the gtktextlayout (otherwise it does not work properly). But at that
point the cursor position is not yet up to date.  We thus need to move
the accessibility cursor notification to after the actual text
suppression, by using another callback.

This fixes cursor position in brltty screen reading.

(cherry picked from commit fa6994d0)
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