Starting from Gtk 3.22 GtkScrolledWindow behaviour was changed to request the
minimum size for its child by default [2]. An API was added to make it
explicitely request the natural size [3].
This was done to have better support out of the box for dynamic content inside
scrolled windows, that is, content that may change allocated size.
When scrolled window content is not dynamic it is best to request the natural
size, especially in presence of wrapping labels, that may otherwise end up
taking a very small width.

Explicitely request natural size allocation to Gtk::ScrolledWindow child when
Gtk >= 3.22.

References:

[1] GNOME Bugzilla - Bug 766569 Better size requisition for GTK_SCROLL_NATURAL children
    https://bugzilla.gnome.org/show_bug.cgi?id=766569

[2] Gtk Commit - GtkScrolledWindow: Make propagation of natural child sizes optional
    GNOME/gtk@0984d162
    "Making propagation of child natural sizes mandatory (or default, even) was
    evidently a mistake as this causes dynamic content in a scrolled window
    to resize it's parent when the scrolled window is competing for space
    with an adjacent widget."

[3] Gtk 3.22 Reference Documentation - GtkScrolledWindow/set_propagate_natural_width/height()
    https://developer.gnome.org/gtk3/3.22/GtkScrolledWindow.html#gtk-scrolled-window-set-propagate-natural-width

[4] Gtkmm 3.21.6 NEWS
    https://gitlab.gnome.org/GNOME/gtkmm/blob/3.21.6/NEWS
    "* ScrolledWindow:
       - Added get/set_propagate_natural_height/width()
         and the properties."