Export module scaling options and full-size caching

[cru]

When using image scaling in export module, it could allow one to choose the method for scaling. So instead of the current method, applying filters to scaled images, which is fast but unreliable, one could have a choice to pick processing at full size and scale down after processing using a selectable method (e.g. Sinc/Cubic/Linear algorithms).

This would also help to work around problems like described in ticket:277 .

A checkbutton for "use full-size cache", causing the export module to cache the results in a lossless format, allowing the export module to run several times without re-processing the whole image, could speed up such operations. E.g. exporting at full size and after that creating a web gallery from the same images would receive a huge performance boost.

Therefor the cache manager needs to reliably lookup cached full-size images, thus it would be reasonable to incorporate cryptographic hashes of both the sidecar file and source image file.

Also means for configuration are required. At least the cache path should be configurable (e.g. to use cheap storage or special location) and a configurable rule how long cached images may live in the cache. Also a "clear cache now" button whould be nice, as well as a "force re-processing" checkbox in the export module.