TextEditor: CodeAssist: Allow immediate proposals

Immediate proposals are displayed for asynchronous providers and fragile
proposals.

The idea is to show already available results immediately. The proposal
calculated in the process runner can then replace the immediate
proposal.

Change-Id: I7903e6677c9dfeb4957eb416062fff1fb01ff23f
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>