BookMap and PageBrowser instances can be stacked; when
toggling a bookmark in a lower PageBrowser and closing it,
make sure the bookmark is shown in the BookMap we get back to.
Also consider TOC and hidden flows as a thing that can be
edited and needs refreshing, so we're ready for next commits
about custom TOC and custom hidden flows.
Also update Reader footer when closing the last BookMap
or PageBrowser.