[USETUP] Improve the formatter & check-disk state machine. CORE-7749

[USETUP] Improve the formatter & check-disk state machine. CORE-7749

- Find or set the active system partition only once, when we start the
  formatting stage. If the system partition is to be created in some
  non-partitioned space, we create it.

- A file-system check is always scheduled for both the system and the
  installation partitions.

- If the system partition was already formatted (which usually means
  that it already existed on the system), don't ask for formatting it.

  CORE-13205

- For each partition to be formatted, the file-system menu list is
  re-created, in order to display only the FSes that are compatible
  with the selected partition, and allow to give the user the choice
  to keep the existing file-system only if the selected partition was
  already formatted.

- Check whether the user attempts to delete the partition on which
  the installation source is present, only if it is not "new" and is
  formatted.

- Check first whether the current partition is the system partition,
  before displaying the CONFIRM_DELETE_SYSTEM_PARTITION_PAGE.