Import session

This pull request will make the "storage" implementation used for tethering and camera import share the same implementation through dt_import_session_t usage. This is also pre-work and needed by the scan view which also needs to handle a session storage. 

*  Moved configuration to preference panel
*  Shared code to test if filename already exists to prevent overwrite
*  Cleaner capture view and camera import implementation due to a lot of 'session' code removed
*  Session storage belongs to camera jobs and not the view
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