Improper handling of album art URLs

Panflute currently treats album art URLs as nothing more than a file name prefixed by "file://". This is wrong and causes the following problems:

1) The applet fails to load files whose URLs have escape sequences in them. VLC, for example, will use spaces in the file names in its art cache, and encodes spaces as "%20" in the URL. The applet looks for a file with "%20" in the name and fails.

2) URLs aren't always file://. VLC initially reports album art with http:// URLs before caching the artwork, and Rhythmbox with the Jamendo plugin always uses http:// for the artwork. The daemon blindly puts file:// in front of the URL from Rhythmbox and the applet then looks for a file actually called "http://...", which obviously fails.

Panflute should use proper URL encoding and decoding instead of blindly adding or removing "file://" from it. It should also handle http:// URLs properly instead of trying to treat them as local files.