JAlbum plugin interface
On start up, jAlbum scans class files ending in *Plugin inside the
"plugins" and "ext" directory (jar files)
If found classes also implement this interface, an instance of that class
is created. The instantiation is performed early during start up, before the
JAlbumFrame and AlbumBean objects are created.
When all critical JAlbum objects are created, jAlbum calls init() on all