projectorganizer: use "folder" icon name instead of "gtk-directory"

This fixes missing folder icons in the sidebar on OS X (and hopefully Windows too).
projectorganizer: verify icon image availability in current theme

GIcon doesn't try to load the icon bitmap, just represents the icon
by MIME type. It may happen the MIME type is registered but the
corresponding icon doesn't exist on the system (happens on OS X,
Windows) in which canse the GIcon cannot be used.