Add feature to not create Image element

Image properties in QQEM create Image QML component automatically into the exported effect component. This is what you usually want. But sometimes it is preferred to not create it automatically if content is created by the code which uses the component. For example effect uses mask which is image in QQEM but in target it is Rectangle/Item that changes dynamically to make it scalable.

Proposal: Add "Create QML Image element" checkbox into image properties. Defaults true, so current behavior. When unchecked, Image elemet is not created and image file does not get copied in export.