QmlJS: Auto completion on existing code parts just inserts

Steps to reproduce:

• create/open a Qt Quick Application
• place cursor amidst a property or item and trigger auto-completion
  • if proposal widget comes up - select something

Current result:

• auto-completion simply inserts the suggestion without taking care of what is already present
• you normally have to remove additional (former present) parts

Expected result:

• skip parts of the suggestion that are already present if possible
• avoid corrupting code when using auto-completion