[SETUPLIB][USETUP] Remove CurrentDisk/Partition from the partlist lib code, and move these into the UI code.
- Add also some validation ASSERTs and simplify the code here and there.
- The installation partition is called "InstallPartition", while the
global "CurrentPartition" is the disk region currently selected in
the partition UI list, on which prtitioning operations are effectued.
- Extend CheckActiveSystemPartition() to use an optional alternative
disk or partition in case the actual system partition (present in the
first disk) cannot be used, e.g. because we don't support writes on it.