CppTools: Fix test_modelmanager_defines_per_project

Change-Id: Ia2e4dc776d3040d2ccb13d9b029c2f0cc8db0fcd
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
TextEditor: Move isAsynchronous() up to IAssistProvider

...from CompletionAssistProvider, so other providers can be executed
asynchronously, too.

Change-Id: I6ec06f6d76bc2937bc272450b4e8dffd81ee868e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>