Better icon pixbuf caching

With the inclusion of tooltip pixbuf previews the icon pixbuf would
get recreated every time the tooltip pixbuf was generated. So now it
will cache both the normal size icon pixbuf and the tooltip pixbuf.
The pixbufs are stored in the base icon so that all the different
icon types don't have to do the same redundant checks.
Icon stays in place when renamed (Bug 1678)
Unselect the desktop icon after activating it. (Bug #8640)
Icon spacing

Fix the spacing the between the icon pix and label. Also calculate
the label radius once rather than all the time.
Monitor the gvfs directory

Added a file monitor watching the gvfs directory since there's no
other way to be notified of these events. Now when the user adds
an emblem to a file and gvfs commits the change to the db, xfdesktop
will update the icons to reflect this change. There is about a one
minute delay from setting the emblem and gvfs changing the file.
Custom folder icons (Bug 8326)

When showing thumbnails on the desktop, xfdesktop will search
inside folder for semi-standard image files to use as the folder
icon (folder.jpg, cover.jpg, albumart.jpg, & fanart.jpg).
Don't call wnck_screen_force_update

Remove the call to wnck_screen_force_update and let wnck initialize
during the main idle loop. Also fix a couple memory leaks.