operations on selections take too long on large collections

[dmoore]

to reproduce: select a handful of files in a very large collection of image (10,000+) and choose an operation (e.g. add a tag). The background task will scan through the entire browser view looking for images that have been selected.

A better way: maintain a list of selected images to avoid the unnecessary searching. (drawback: the list of selected images wastes memory, and must be kept free of duplicates, which uses CPU time each time the user changes the selection)