Applications need a way to fix or adapt the decoration layout,
for situations like split header bars. Setting the layout from
the theme with a style property did not offer a good way to do
this, and the ::show-close-button property does not provide
fine-grained control.
To improve the situation, move the layout string to a property of
GtkHeaderBar which is backed by a setting. This allows platforms to
set a default button layout independent of the theme, while applications
can override the default.
The style GtkWindow style property is now deprecated and ignored.
It is a fairly common case to just want a title, and not
reserve extra space for a subtitle. This is much easier
to get right by setting a boolean property than by
constructing a custom title widget.
Allow showing the fallback app menu with a menu button
in the header bar. Applications have to explicitly enable
this by calling gtk_header_bar_set_show_fallback_app_menu.
Add a boolean property that controls whether a window close button
will be shown in the header bar or not. Doing this in the toolkit
will ensure consistency of the visual apperance.
https://bugzilla.gnome.org/show_bug.cgi?id=702971
