Cache full schematic sheet list sorted by sheet page number.

The sheet list is returned as a copy of the cached list rather than a
reference to prevent external code from changing the list.  While not as
performant, it eliminates the risk of the sheet list being altered in
ways that could break the schematic.  The sheet list should only be
updated by calling SCHEMATIC::RefreshHierarchy() when any appropriate
sheet changes are made.

Note to developers: there is something inherently different about how the
QA tests are loading and handling schematics versus the schematic editor.
Using the cached sheet list for the SCHEMATIC object will cause some QA
test to fail.  This is why SCHEMATIC::Hierarchy() has not replaced
SCHEMATIC::BuildSheetListSortedByPageNumbers() everywhere.

eeschema/sch_screen.cpp

@@ -1696,7 +1696,7 @@ void SCH_SCREENS::ClearAnnotationOfNewSheetPaths( SCH_SHEET_LIST& aInitialSheetP
 
     // Search for new sheet paths, not existing in aInitialSheetPathList
     // and existing in sheetpathList
-    for( SCH_SHEET_PATH& sheetpath : sch->BuildSheetListSortedByPageNumbers() )
+    for( SCH_SHEET_PATH& sheetpath : sch->Hierarchy() )
     {
         bool path_exists = false;
 
@@ -1836,7 +1836,7 @@ void SCH_SCREENS::UpdateSymbolLinks( REPORTER* aReporter )
 
     wxCHECK_RET( sch, "Null schematic in SCH_SCREENS::UpdateSymbolLinks" );
 
-    SCH_SHEET_LIST sheets = sch->BuildSheetListSortedByPageNumbers();
+    SCH_SHEET_LIST sheets = sch->Hierarchy();
 
     // All of the library symbols have been replaced with copies so the connection graph
     // pointers are stale.
@@ -1935,7 +1935,7 @@ void SCH_SCREENS::BuildClientSheetPathList()
     for( SCH_SCREEN* curr_screen = GetFirst(); curr_screen; curr_screen = GetNext() )
         curr_screen->GetClientSheetPaths().clear();
 
-    for( SCH_SHEET_PATH& sheetpath : sch->BuildSheetListSortedByPageNumbers() )
+    for( SCH_SHEET_PATH& sheetpath : sch->Hierarchy() )
     {
         SCH_SCREEN* used_screen = sheetpath.LastScreen();