1.5.2: Sort according to capture time - a cache only option

The new sort according to capture time is a great feature. Here is an improvement to make it even more practical: make it use cache only, or use cache when EXIF is not available, or make a cache only option.

Currently, the sort function first extracts EXIF date again, and when the picture does not contain EXIF data, extracts file creation date. Such dates are then used for sorting.

An album may contain both digital photos and scanned pictures. In this case, the sort result is inaccurate because file creation date of a scanned picture is rarely photo capture date.

On the other hand, EXIF date (or file creation date in lack of EXIF) is always cached in G1 data file. This cached date is not only visible, but can be modified via G1's edit_caption.php interface. For information purposes, I have always edited the cached date of scanned pictures to reflect the actual capture date. (I also checked that sort function does not rewrite this cache.) It would be really nice if we can use this edited data to sort pictures.

Each of the proposed methods makes some assumption about existing data and thus has merits and drawbacks. For my purpose, use cache when EXIF is not available would be least effort.