Move `state.json` and .current restoration into GenericChannel.gui().
Current category is reselected by TreeView traversal on instantion now.
Previous state now load through config.state() for channels/__init__,
not in main/init_app_state anymore (just row:expand / winsizes now).
Disable .currentcat() overwriting, redundant now in display_categories().
Still need to avoid second .select_current() call in first_show().