We finally have the new disk menu mostly working

Finally we have the new disk menu (mostly) working on the MEGA65:

This is no small milestone, because it doesn't touch the SD card directly at all.

It uses Hypervisor calls to do everything: to check that it is running on a MEGA65, to list the files on the SD card, and then to ask the Hypervisor to mount the disk image, which in turn does some checks to make sure the disk image is fit for mounting. We can now proceed to implementing more Hypervisor calls with confidence.

Also, by using the Hypervisor, the program is able to be quite compact: less than 2KB, despite having a full screen browser to select the disk images, and a (currently disabled) sort routine to show the images in the right order. Indeed, almost 1/4 of the size is text messages.

This now sets the scene for us to progress all of the other previously blocked progress on the Hypervisor, to make the whole machine pleasant to use.