Refactor and rename GParted_Core::open/close_device_and_disk()

These functions in GParted_Core:
    open_device()
    open_device_and_disk()
    close_disk()
    close_device_and_disk()
call the following functions in the libparted API:
    ped_device_get()
    ped_disk_new()
    ped_disk_destroy()
    ped_device_destroy()
which don't open or close anything.  Instead they allocate and
deallocate PedDevice and PedDisk memory structures which describe block
devices and partition tables respectively.

Rename functions:
    open_device_and_disk()  -> get_device_and_disk()
    close_device_and_disk() -> destroy_device_and_disk()
and merge open_device() and open_device() as each only wrapped one
libparted function and was only called from a single place.