Feature request: provide way to autopopulate Valid Values from children

[allaway]

For example, we have attributes defined for a parent tissue with children:

medial dorsal nucleus of thalamus, posterior inferior parietal cortex, midbrain, cerebral cortex, frontal lobe, hippocampus, nerve tissue, dorsolateral prefrontal cortex, anterior cingulate cortex, frontal pole, parahippocampal gyrus, superior temporal gyrus, inferior frontal gyrus, cerebellum, occipital visual cortex, inferior temporal gyrus, middle temporal gyrus, posterior cingulate cortex, temporal pole, precentral gyrus, superior parietal lobe, prefrontal cortex, amygdala, caudate nucleus, nucleus accumbens, putamen, temporal cortex, orbitofrontal cortex, ventrolateral prefrontal cortex, medial frontal cortex, primary motor cortex, primary somatosensory cortex, posteroinferior parietal cortex, primary auditory cortex, posterior superior temporal cortex, inferolateral temporal cortex, primary visual cortex, amygdaloid complex, striatum, cerebellar cortex, serum, plasma, splenocyte, blood, primary tumor, Not Applicable, embryonic tissue, meninges, forebrain, medial orbital frontal cortex, medial prefrontal cortex, inferior temporal cortex, middle frontal gyrus, cortical plate, VZ/SVZ, dorsal pallium, bone marrow, Buccal Mucosa, Dorsal Root Ganglion, unspecified, whole brain, Buffy Coat, frontal cortex, olfactory neuroepithelium

Each of these children is defined as it's own attribute as well, with parent tissue. It would be nice to automatically define these as valid values (say, if the valid value property for tissue was left blank), rather than every time we want to define a new tissue to have to add a new attribute and add it to the list of eligible valid values.