Show multiple bootloader devices on the Manual Partitioning screen (#2093793)
    
    Don't keep bootloader devices in a dictionary of mount points. Otherwise, we
    can loose information about bootloader devices of the same type (like biosboot)
    and show only one of them on the Manual Partitioning screen in GUI.
    
    All devices used by an OS installation are now represented by the `devices`
    property of the `OSData` structure. The list of devices includes bootloader
    devices, mount point sources and swap devices. The `swap_devices` property
    was removed.
    
    (cherry picked from commit 60a7960398af0233ebb6877569ec75c66c77e24b)
    
    Resolves: rhbz#2093793