[SETUPLIB][USETUP] Partition code improvements.

[SETUPLIB][USETUP] Partition code improvements.

- SETUPLIB and USETUP: Convert PARTENTRY::DriveLetter to WCHAR.

- SETUPLIB: Retrieve volume label.

- SETUPLIB and USETUP: Move the partition types (IDs) table from
  USETUP to SETUPLIB so that they can share it with the 1st-stage
  GUI installer too. Also take the opportunity to update the table
  with information from http://www.win.tue.nl/~aeb/partitions/partition_types-1.html
  and cross-checked with the list from Paragon Hard-Disk Manager.

- USETUP: Simplify PrintPartitionData().
- Add some code comments.