Collapse navbar unless window is really wide

[henrikt-ma]

This PR addresses the problem that when HTML documentation is viewed on a small screen, too much horizontal space is occupied by the navbar on the left side. The solution is to collapse the navbar unless the window is really wide, expanding it when hovered. Readers who prefer to have the navbar open all the time to see the overview of the current chapter just have to make the window sufficiently wide.

Alternative approaches that were considered include:

• Reducing the fixed width to much less than the current 350px is not good because truncated section titles don't look good, meaning we don't wan't even more of them.
• Allowing line wrap is also a bad alternative since section titles occupying several lines can easily make the table of contents expand in vertical direction so much that it's hard to get an overview.
• An open/close button for changing the state of the navbar. This would be meaningful if there was really some state of the navbar that was worth preserving when jumping between chapters, as this seems to require cookies to avoid a state that gets reset every time one jumps to a different page (chapter). We don't seem to have enough state of interest to motivate introduction of cookies.

[henrikt-ma]

In really narrow windows (small screens), I'd actually prefer a navbar that occupies no horizontal space at all when collapsed, allowing the main content to begin at the left edge of the window. I think it would remain to be seen whether accessing the navbar from a small icon in the top left corner would be convenient enough to also replace the proposed solution on medium sized windows.

[henrikt-ma]

I haven't tried this on a device with touch screen. While it might seem as it wouldn't work, I note that examples of hover-expanded menus seem to work well on my phone. Therefore, unless someone knows that relying on hovering is a really bad idea, I suggest that we merge this so that we can test it ourselves on various devices before abandoning CSS hover effects in favor of javascript.

[HansOlsson]

Hmm...
I tried to check it and downloading the generated files didn't show any collapsing navbar regardless of window width (on iPhone it was just on top). It could be a problem with using css-files from local file-system (I have noticed this before).

But at least it doesn't seem to cause problem, so I will just check it a bit more and then accept and merge.

[henrikt-ma]

OK. If it doesn't work to satisfaction after the merge we shouldn't hesitate to revert the merge.

It also makes sense to not explore a solution for really narrow screens before we've had a chance to see the current design in an actual build.

[HansOlsson]

Ok, seems ok and doesn't seem to break everything.
Let's try it.