Before Gtk 3.22 GtkScrolledWindow propagated natural size to its child
and so on to descendants. In Gtk 3.22 this was changed to always
request the minimum size. This was done because it is believed to be
a safer default (gives a better behaviour) in case of dynamic content
inside the scrolled window, that is, content that may change allocated
size. [1][2]

When the scrolled window content is not dynamic the natural size is
preferable because it gives a better looking layout and without any
downside.

In the case of GParted content is not dynamic, so request the scrolled
windows to allocate children at natural sizes for Gtk >= 3.22.

The benefits of natural size allocation are evident in presence of
wrapping labels (for example inside the "Partition Info" dialog), that
with the minimum size request likely end up taking a very small width.

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