Also removes the side-effect that SCH_SHEET_LIST's
c'tor would sort the list (and write virtual page
numbers) anytime the starting sheet was the root.
Also, definitely don't build a SHEET_LIST (sorted or
otherwise) if you're not even going to use it.
Also don't build SCH_SHEET_LISTs on idle events. Better
to just always have the Next Sheet button enabled (we
already beep if you click it and there's no next sheet).
Also, use a SCREEN_LIST when you can. It's much cheaper
to create.
- Move PLUGIN_FILE_DESC to common.
- SCH_PLUGIN: rename Load -> LoadSchematicFile, Save -> SaveSchematicFile.
- Use PLUGIN_FILE_DESC and CanRead* in schematic plugins.
- Return none/unknown types from Find/GuessPluginType functions.
- Iterate over file types for file wildcards.
- Clean-up header checking in IO plugins.
- Use PCB plugin list in IO_MGR::GuessPluginTypeFromLibPath.