HdyViewSwitcher: Show the window title

When @zbrown submitted the view switcher, it had a third mode which was the window's title, but I wasn't happy about it as that title had to be set manually. Instead, I introduced HdySqueezer to handle that externally.

I now feel I was wrong and the view switcher should by default shrink to display the window's title, making the squeezer useless for that, but at the condition the window title is automatically retrieved like GtkHeaderBar does. Of course, the HdyViewSwitcherBar shouldn't show the window's title.

@zbrown