Support updating window buttons/icon from HdyHeaderBar
This is a GTK issue that's not even exclusive to headerbars inside the window (happens with split headerbars etc), but would be nice to fix it regardless.
We should trigger layout update when window's deletable
, resizable
, modal
or transient-for
properties change, trigger icon update when icon-name
changes.
There's also the title, but that one is probably fine.