TextEditor/CodeAssist: Limit automatic proposals only to completions

...since it does not make sense for TextEditor::QuickFix and
TextEditor::FollowSymbol.

Regarding the bug:
- The bug was exposed by having "Active completion" set to *not*
  "Manual" in Options > Text Editor > Completion.
- After choosing one item from the overrides list the cursor was
  positioned just right before the beginning of a function name, which
  is a valid activation sequence (see CodeAssistantPrivate::process()))
  As a consequence, a new proposal was requested.

Task-number: QTCREATORBUG-10345
Change-Id: Ib0d10192863fb33c0958a9afbf76ca8e7de772f8
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppEditor: Fix crash on virtual function override lookup

...involving templates.

Task-number: QTCREATORBUG-10496

Change-Id: I485d691043f27b03e120353f0dfd6a2fb0aa07ec
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppEditor: Clean up cppvirtualfunctionassistprovider.cpp

- Moves the findMatchingClassDeclaration() call from
  FollowSymbolUnderCursor::findLink into VirtualFunctionsAssistProcessor
  since we already have a SymbolFinder there
- Make canLookupVirtualFunctionOverrides a class member because we plan
  to add some methods
- Better parameter names/order for FunctionHelper::overrides()

Change-Id: I0a93ff5445352d47e808adad45485e520f06946e
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>