* prepare back end for processing additional settings from modal import dialog
* log warning if deletion of original failed
* localizations limited to English

* ImportPhoto::photo: new mandatory argument `import_via_symlink` before optional ones
* sanity check for incompatible settings
* Exec: eliminate `skip_duplicates` (existing `force_skip_duplicates` is enough and takes precedence)
* Create: collapse `if`s to fix smell

* `resync_metadata` did not update keys with value null (but should have). in particular, this prevented updates to `takedate` if the corresponding entry in the DB was set to zero
* too many false positives were caused by comparing int's to their corresponding string representation.

* use session storage to process asynchronous cancel request from GUI during import from server