Thumbnail cleanup UI patch (bug 150483)

[John Spray <jcs116 york ac uk>]

Hi,

http://bugzilla.gnome.org/show_bug.cgi?id=150483

Since we're at the start of a development cycle, perhaps now is a good
time to consider the issue of accumulation of thumbnail files consuming
disk space indefinitely.  At present, when a thumbnailed file is
deleted, its thumbnail file in ~/.thumbnails lingers.  This can lead  to
consumption of substantial wasted space, particularly for those who
frequently handle large numbers of images.

On this bug, I propose and implement a user interface that provides two
functions to help with this problem:
      * "Empty Thumbnail Cache" deletes all thumbnails
      * "Clean Up Thumbnail Cache" deletes only those thumbnails that
        are for files that no longer exist.

These are made available as buttons in the nautilus File Management
Preferences dialog.  Although I recognise that this is not an incredibly
elegant way of dealing with the problem, it at least provides some
respite, and a definite improvement over the present situation (which
demands that the user know about ~/.thumbnails).

Some other possible mechanisms for dealing with this problem that others
may be interested in:
      * Some kind of indication to the user when they're low on disk
        space, the thumbnail cleanup being presented as one way of
        freeing space.
      * The automatic background removal of redundant thumbnails.  This
        involves opening thousands of pngs, so would have to be
        implemented very delicately to avoid impacting performance.
      * Intelligent removal of thumbnails by nautilus upon
        deletion/moving of files.

Kind regards,
John Spray
(jcs116 york ac uk)