Adapt to new kdevplatform API regarding assistants

Fixup the existing AdaptDefinitionSignatureAssistant, fixup
initialization in CppLanguageSupport.

Add a few more tests for assistants

REVIEW: 118543
Rename: AdaptDefinitionSignatureAssistant

Rename to AdaptSignatureAssistant, streamline filenames
Add a test case for Bug 335803

CCBUG: 335803
REVIEW: 118627
Add a CMakeProjectVisitorTest for bug 335803

CCBUG: 335803
REVIEW: 118627
Do not crash unit test.

BUG: 335989