Correct icon-grid width under GTK3

When opening multiple windows, the Taskbar plugin does not resize the
taskbar buttons if they exceed the available space. Everything to the
right of the taskbar slides off the screen instead.

Do not set the minimum width to the same value as the natural (i.e.
current) width if the width is constrained by the allocated space,
but set it to the smallest possible value.

This likely fixes github issue #23, reported by FredericGuilbault,
fixes github issue #29, reported by johnfound,
fixes github issue #63, reported by LiohMoeller,
fixes part of https://bugs.debian.org/1052050,
fixes https://bugs.debian.org/1052376, and
fixes part of https://sourceforge.net/p/lxde/bugs/968.

(commit message amended by committer)

src/icon-grid.c

@@ -392,7 +392,7 @@ static void panel_icon_grid_get_preferred_width(GtkWidget *widget,
     }
     panel_icon_grid_size_request(widget, &requisition);
     if (minimal_width)
-        *minimal_width = requisition.width;
+        *minimal_width = ig->constrain_width ? 0 : requisition.width;
     if (natural_width)
         *natural_width = requisition.width;
 }