Highlight selected page
Addresses one request from #248, and I believe it resolves #9 (closed).
I use a hardcoded color for now, which looks good on light themes and ok on dark themes. The theme's accent color would be a better choice but I've decided that some highlighting is better than none after spending 2 hours trying to figure out what the clean way of getting that color from GTK is.
Edited by Jan Schejbal