Support CSS a little better than before
With these changes, GtkHeaderBar has a complete CSS box support. We should likely make a mixin based on that and use it everywhere to fix things like #229 once and for all.
But for now just fix the immediate issue and support min-height/width and box-shadow for HdyHeaderBar and HdyTitleBar. And also box-shadow for HdyAvatar because everybody loves unrelated random changes in MRs