Libraries have been 100% case-sensitive for a while now; there is no longer a
need to keep this option around. This will change nothing except for any
stragglers still manually specifying this old option.
* Add sheet number that is set by schematic file load order. This duplicates
the previous SCH_SHEET_PATH behavior.
* Uncouple SCH_REFERENCE and SCH_REFERENCE_LIST from SCH_SHEET_PATH.
* Add - operator to SCH_SHEET for comparison purposes. This duplicates the
behavior of SCH_SHEET_PATH::Cmp().