display-page: Use a less hackish way to center title
display-page: Use a less hackish way to center title

Currently we use two toolbars layed on top of each other, one for
the tool items on the left/right and one for the centered title.
The centering can be achieved with a size group, which feels a lot
cleaner.

https://bugzilla.gnome.org/show_bug.cgi?id=666354

vbenes/gnome-boxes,vbenes/gnome-boxes