* Move code from function ClearProjectDrawList() into SCH_SCREEN object
  Clear() method.
* Delete eeschema/delsheet.cpp as none of the code in it is ever called.
* Move global spice and net list command line variables into SCH_EDIT_FRAME
  object as priviate members and provide access methods.
* Remove unnecessary header includes from eeschema/sheet.cpp.
* Minor coding policy fixes.