Add sparsebundle support

[vasi]

Since a couple of years ago, BasiliskII and SheepShaver have supported sparsebundle format hard drive images. Medusa currently treats these as directories, so they can't be selected. It would be great if they were supported.

[vasi]

Just for giggles, I manually edited Medusa.storedata to include a sparsebundle. It works well! The only problem is that the "size" column for disk images is too small, so sizes over 2GB display wrong.