CppEditor: Do not cancel highlighter on user interaction

e.g. changing cursor position or editing text.

CPPEditorWidget::updateUses() does not depend on the highlighter, but
only on the semantic info update.

Task-number: QTCREATORBUG-11367
Change-Id: Ia9f5ed7c2adc6899995a5d966a20e45b2f87d9e3
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>