In Gtk 3.22 GtkScrolledWindow behaviour was changed to request the
minimum size for its child [1]. An API was added to make it request
the natural size [2].
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.
If the scrolled window content is not dynamic it is better to request
the natural size because it gives a better looking layout. This is
especially true in presence of wrapping labels, that may otherwise
end up taking a very small width.

Request natural size allocation to child of Gtk::ScrolledWindow for Gtk
>= 3.22.

References:

[1] Gtk commit from 2016-08-31:
    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."

[2] Gtk 3.22 Reference Documentation - gtk_scrolled_window_set_propagate_natural_width
    https://developer.gnome.org/gtk3/3.22/GtkScrolledWindow.html#gtk-scrolled-window-set-propagate-natural-width

[3] 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."

Closes !39 - Always request natural size inside Gtk::ScrolledWindows