Function for automatic stack detection

[Felix Hagemann]

After the introduction of stack numbers and the associated functionality, one thing to make this easy to use is an automatic detection of stacks as match-n-shift does it. At the moment manually assigning stack numbers in the Camera and Lens tab is slow and a lot of work for larger projects.

Using Date/Time Original and Shutter speed from the Exif values, it should be possible to do a fairly good job.

[tmodes (tmodes)]

Implemented in changeset ca5f2a72cd09.