[NB] [Suggestion][UI] Document/scrivenings button icon toggle

Document/Scrivenings button currently toggles between white and black page icon, rather than between single page and and stack icon as discussed in tutorial.

Scrivener beta (19429) 20 Nov 2017
Windows 10 Pro 64 bit 10.0.15063

P.S. Hmmm, I see that Scrivener 3 on Mac OS X employs the stack icon for this button and just toggles between white and black version of the stack icon. So, I guess disregard this. Though I prefer single page vs stack.

Aside from updating the icons, the way this works hasn’t changed from 1.9.7. If Scrivenings is an available mode (i.e. you are viewing a container or a multiple selection), it always shows the stack icon; otherwise, it shows only the single document icon. That button then toggles between selected (dark grey) or deselected depending whether Scrivenings view is currently enabled.