Support iOS 10 properly

[TimOliver]

For the time being, I've had to raise the minimum requirements to iOS 11.

It turns out that UINavigationBar was drastically changed in iOS 11. Before that, there was no UINavigationItemContentView that could be conveniently hooked.

Unfortunately, initial tests with iOS 10 seemed to indicate that it's not easy to isolate the title label and suppress it for the opening transition.

I'd like to play with it more when I get the time, but since iOS 10 will be deprecated once iOS 12 hits, it might not be worth the effort.