CMS-3852 ContentWizardStepForm : two 'ImageUploader' appears, when Image...

1. added check for existing  of attachment, new ImageUploader will be added if there is no a attachment.
2. when image changed in InputOccurenceView(new image uploaded ), generated only a "api.form.inputtype.support.ValueChangedEven" , ValueAddedEvent generated in the previous version.
3. when image deleted, generated ValueRemoveEvent
4. check for null added