HeaderLabel: Expose label xalign property

Problem

I cannot properly, fully center the placeholder text here. The wrap is done assuming the text is left-aligned, leaving the placeholder here strangely bungled

the style class being deprecated, it would be nifty to use headerlabel here, but it lacks the granularity of Gtk.Label

Proposal

expose the base Gtk.Label xalign property for Granite.HeaderLabel I suppose one could want the wrap property too to control how it is displayed

current workaround is to

var a = (Gtk.Label)placeholder.get_last_child ();
a.xalign = 0.5f;

Prior Art (Optional)

No response