Allow Selection of SmartPort disk images

[samkusin]

This issue came about in response to some usability concerns mentioned here: #79.

Currently there is only one hardcoded Smartport disk image recognized in the emulator. It should be somewhat easy to make this selectable.

The challenge is that we probably don't want to import 32MB disk images into the user's library. Or if we do, then we'll need to copy these to the user's disk library.

We may also want to save the disks loaded on the disk tray in the config so that they're already there when running the emulator again.

Finally we may not want to restrict "Import Disk" to the disk type of the selected slot but instead consolidate all import disk options into a single option. That may require some rework of how we generate directory listings of disks.