CHANGED: The text variable ${SHEETNAME} now always represents the name of the
sheet when used anywhere in the schematic editor, including the title block
ADDED: A new text variable ${SHEETPATH} which is replaced with the path to the
current sheet - e.g. "/Sheet 1/Sheet 2".
To ensure backward compatibility, the default drawing sheet now uses
${SHEETPATH}. Custom drawing sheets will need to be manually edited.
Keeping them in base_units means that we can't introduce
any dependence on these functions to anything that needs to
compile without one of the unit defines (EESCHEMA, PCBNEW, etc)