QmlJSEditor: Simplify usages highlighting

There is no need to fire a timer over and over again if semantic info is
not up to date. Instead, update usages whenever cursor position changes
and semantic info is up to date, and whenever semantic info is updated.

Change-Id: If2d8fe2a0211b9d659bea5c747704380a64cc1ca
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>