[ptx] new "Time Series" feature enabled

[Ed Halley]

Similar to one of Max Lyons' scripts, I added an automatic time-series
feature to the Images tab.  This should make it easy to pick out a
panorama series from the middle of a large photo shoot with many
unrelated images.

 * optionally add one or more images to an empty project

 * choose "Add time-series of images..."
   (if you had no images, it offers you a dialog to pick a starter)

 * the subdirectory of each loaded image is scanned for JPEG EXIF info

 * any image originally created (shot) within 60 seconds of any of
   the loaded images is then loaded too, and so on, to find the series

The only downside to this, so far, is that the images are found and
loaded in random order.  This doesn't bother the software, but it's not
as convenient to find the adjacent images in the Control Points tab.

I will probably add the extra complexity to "sort" the loaded images by
EXIF creation time, but I thought I'd open up the feature to comments on
the feature and the code now.

(Two other minor invisible modifications included today:  a segfault on
some malformed JPEGs averted in jhead, and a simplification of
OnSaveProjectAs(), as discussed with Pablo earlier.)

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