Use other timestamps if there is no taken timestamp

[Felix-Franz]

Is your feature request related to a problem? Please describe.
If a picture has no taken timestamp the scanning timestamp will be used.
So old images may be displayed higher up than current.
Mostly the timestamp was not set correctly on old camera models and so mostly old pictures are the ones without the taken timestamp.

Describe the solution you'd like
Try to use another possible timestamp like the file created timestamp or the modified timestamp if the created timestamp does not exist.

Describe alternatives you've considered
Since the images are mostly old ones without a taken timestamp it might make sense not to use the scan time as timestamp but a time from the past so that the images are always displayed at the bottom (e.g. 1.1.1900).

Additional context

Screenshots of the Windows file properties

[damianmoore]

Thanks @Felix-Franz. Another good suggestion.

[GyanP]

Task done and changes pushed on branch 249-img-date-taken

[damianmoore]

Merged as of 9df9675. Will be in next release.