Support overlay (or arbitrary widgets) as child of HdyPreferencesWindow
I'm looking into adding a Plugins page to Geary's prefs and would like to show an in-app notification in teh prefs dialog when a problem occurs loading a plugin, I can't do that however since I can't add a GtkOverlay as a child to HdyPreferencesWindow:
Can't add children of type GtkOverlay to ComponentsPreferencesWindow