When enabled in BookMap, both BookMap and PageBrowser get:
- alternating gray background on chapter spans
- hatched background instead of uni-gray on hidden flows.
Also, with both normal look and this alternative theme,
show some hatched overlay on thumbnails part of hidden flows.
ReaderToc: for each TOC item, have it carry its sequence/number
in that level (we do this in validateAndFixToc() where we are
already iterating all the items).
c51b0c7bb9