- 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.