action-bar: Use dynamic label to show page count

In the mockups for the mobile interface, a dynamic icon is used that shows the number of open pages. We were using a static icon, meaning that users had no frame of reference for how many open tabs there were.

Now we use a dynamic label with custom styling to look like an icon.

Out of necessity it includes some changes to the colors of the incognito header, as the css seems to have not been generated since the Adwaita changes. If this is not acceptable I can submit those changes separately and then rebase on top of that once merged.