Further fancy sidebar features, now with in-situ updates.

Created new feed-action.php for handling updates of feeds in place from sidebar.

Replaced hard-coded image assets with map, defaults set in fof-asset.php, and overridable in fof-asset-custom.php.

Added image/tag-icon.svg ( sourced from http://commons.wikimedia.org/wiki/File:Black_book_icon.svg ) as a default tag icon.  Added image/spinner.gif and image/warn.gif from unknown sources.

Moved feed table row generation from sidebar.php into fof_render_feed_row() in fof-main.php, to be accessible via ajax.