headerbar: do not show buttons for modals/transients

headerbar: do not show buttons for modals/transients

GtkHeadeBar checks the window type hint to determine if the regular
buttons such as menu, maximize or iconify should be visible in the
header bar.

However, an application may very well use a "normal" toplevel window and
set it transient and modal afterwards. In such a case, the iconify
button would remain visible, and the user can hide the window, but being
a modal, the parent window would remain insensitive.

Check for the window type, modality and transient relationship to decide
whether or not the regular toplevel buttons should be visible in the
header bar.

https://bugzilla.gnome.org/show_bug.cgi?id=767052