Issue #12491: Fix GimpUnitComboBox immediately closing on certain platforms

I noticed that the issue of the combo box popup getting immeidately
closed on Linux X11 (and probably macOS as well) only happens when using
GimpUnitComboBox as used in gimppropwidgets.c and gimpsizeentry.c while
there is no issue when used in gimpstatusbar.c

The only noticable difference I saw was that in status bar was that
the wrap width of combo box was set to 1 via
gtk_combo_box_set_wrap_width. Which was added as a hack for another
issue.

I did the same for the other two uses of GimpUnitComboBox and the issue
disappeared so I moved this hack to GimpUnitComboBox itself as it seems
to be generally useful.

The change doesn't make much sense to me and is a workaround for
something that should probably be changed in GTK but well.

As explored in !2654 I think we shouldn't use GtkComboBox but reimplement
using a Popover. This would get rid of these workarounds as we wouldn't rely
on internal behaviour of GtkComboBox.

Couldn't test macOS or Windows but Linux X11 and Wayland work just fine.
Tested by: GDK_BACKEND=x11 gimp and GDK_BACKEND=wayland gimp

Fixes #12491

Superseedes !2654

app/display/gimpstatusbar.c

@@ -308,9 +308,6 @@ gimp_statusbar_init (GimpStatusbar *statusbar)
   statusbar->unit_combo = gimp_unit_combo_box_new_with_model (store);
   g_object_unref (store);
 
-  /* see issue #2642 */
-  gtk_combo_box_set_wrap_width (GTK_COMBO_BOX (statusbar->unit_combo), 1);
-
   gtk_widget_set_can_focus (statusbar->unit_combo, FALSE);
   g_object_set (statusbar->unit_combo, "focus-on-click", FALSE, NULL);
   gimp_statusbar_add_size_widget (statusbar, statusbar->unit_combo);

libgimpwidgets/gimpunitcombobox.c

@@ -77,6 +77,9 @@ gimp_unit_combo_box_constructed (GObject *object)
   G_OBJECT_CLASS (parent_class)->constructed (object);
 
   gimp_unit_combo_box_popup_shown (combo);
+
+  /* Hack to fix issue #2642 and #12491 */
+  gtk_combo_box_set_wrap_width (GTK_COMBO_BOX (combo), 1);
 }
 
 static void