Completions: move continuations upper in proposals list.

Since fuzzy completions are allowed, the lexicographically first proposal
is not necessarily most relevant. The patch modifies sorting of proposals
so that the exact match and continuations go first, and fuzzy completions
follow.

Moreover, being a continuation seem to be a more important characteristic
of a proposal, than being it a function argument or keyword etc. That's why
the check for continuation is placed before the check for order.

Task-number: QTCREATORBUG-8737
Task-number: QTCREATORBUG-9236
Change-Id: I89aae9d2ce6bfa59af7c2f75e6f3af00212008ca
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Alexey Zhondin <lexxmark.dev@gmail.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>