Any element creation should set its containing resource as non imported

[gdaniel]

In #907 we ensure that creating an element from the New object tool in the explorer sets its containing resource as not imported.
This should be extended to any ways SysON provides to create elements, not the New object tool only. For example, this should be extended to tools in diagrams that create elements.

A possible way to do this is to update the SysONEContentAdapter to remove the imported flag when a new element is added in a resource. This implies to have the capability to disable this feature when loading resources, and when importing new resources via the Upload tool.
The former is already possible thanks to IEditingContextProcessor#preProcess() and IEditingContextProcessor#postProcess(), but Sirius Web doesn't provides these hooks when uploading a model, which are required to support the latter.