Improve code-assist API isSortable

Now IGenericProposalModel::isSortable also takes the current prefix
as a parameter. This is particularly useful for cases like in the C++
completion: For global completions it will only sort if there's
already a content prefix - Notice, however, that the "sorting threshold"
for a maximum number of items is still kept in IGenericProposalModel::sort.

Task-number: QTCREATORBUG-6404
Change-Id: I3c2f282aa565287b8b4989930b6a27093862ef25
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>