Add QAbstractItemModel::flagsChanged (and possibly roleNamesChanged)

There is currently no way (short of a model reset) to signal that the role name mapping or an item's flags have changed.

QItemSelectionModel has lots of logic in its getters to keep the selection set up-to-date, by rechecking that none of the selected elements didn't become un-selectable or disabled in the meanwhile.

Changing role names is much less frequent (one could claim, almost never happens), yet it's a possibility, so there should be a signal for it that doesn't require a model reset.