move string properties normalization into WikibasePage#__normalizeData()

away from editLabels, editDescriptions and editAliases,
thus allowing to call it from other functions

== use cases ==

currently, to edit some labels, the only valid syntaxes are:
* editLabels({'lang': 'val'})
* editEntity({'labels': {'lang': {'language': 'lang', 'value': 'val'}}})

this change adds the following possibilities:
* editLabels({'lang': {'language': 'lang', 'value': 'val'}})
* editEntity({'labels': {'lang': 'val'}})

Change-Id: Iba1fdce1cc199cda0b7f83febdf48ce6a11100e0
Merge "move string properties normalization into WikibasePage#__normalizeData()"