Add/Remove applications support

Add support for using gnome-app-install to add and remove software from a USB disk. This may involve the following steps:
 * Mount the squashfs with a union.
 * Run gnome-app-install in a chroot of the squashfs.
 * Make a new squashfs from the union.

If possible, this feature should account for insufficient free space by making the union only as large as the USB disk. This has the downside of requiring at least the size of the USB disk free on the root filesystem.