Ignore text nodes when parsing XMLAnnotation's value (rebased onto dev_5_0)



This is the same as gh-853 but rebased onto dev_5_0.

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This fixes an error parsing Modulo annotations (e.g. FLIM-modulo-sample.ome.tiff) in Matlab.

This does not fix the problem noted when viewing in Insight, namely that the sliders and do not correctly update the image indices (which I believe is an Insight bug).

/cc @imunro, @sbesson