Move assistant infrastructure to kdevplatform

Noteworthy changes:
* StaticCodeAssistant (from CPP plugin) is now StaticAssistantsManager,
now lives in kdevplatform, so all languages can benefit from it
* RenameAssistant was moved to kdevplatform, as it it useful for all
languages
* BasicRefactoring now has new virtual methods that control the behavior
for renaming actions
LanguageSupport got a new property: refactoring (so languages can expose
their custom BaseRefactoring implementation)

New API:
* New base class for "static" assistants: StaticAssistant
Static assistants exist during the whole session
They are notified when documents change and create solutions based on
that. Current implementations: AdaptDefinitionSignatureAssistant (CPP
language) and RenameAssistant (kdevplatform)
* New manager class: StaticAssistantsManager:
Tracks the StaticAssistant instances, takes care of notifying them
about changes in the editors. Single entry point for registering
StaticAssistants.
Entry-point: LanguageController::staticAssistantsManager

REVIEW: 118542