Improve style changes for the desktop

This patch changes how style-set events are handled. Instead of
re-creating the icon view to deal with XSettings manager changes,
we now reset all the styles on the icon view and let it re-set them
to what it needs to. This stops the icon view from destroying and
recreating all the icons which should make startup a little faster.
Improve cover art loading

Moved the code to the regular file icon section. It now monitors
folders for the album art image and loads it if it is added.
Point to the same initial help page
Release the file monitor
Always depend on dbus-glib (Bug 10745)

Thanks to Samuli Suominen for reporting the issue and proposing
the fix.
Fix build errors

When some flags were passed xfdesktop failed to build because
libexo wasn't being properly added.