Fix #118: Web browser window resize, "bookmark" location is saved+restored several times (throttled): actual reading position gets lost in the page spread

#118 
When resizing a window a bookmark would be saved and restored multiple times per resize call.
With this fix the bookmark only gets saved at the start of a series of browser resize events, the original bookmark gets restored at each throttle interval, and at the trailing end.

Although I've tweaked this fix for the best behaviour, some tweaking might still be required to perfect it,