MIgrate GtkBox packing properties to what is supported in both GTK+3 and GTK+4
This change removes the GtkBox packing properties that are removed in GTK+ 4, thus allowing us to more easily shift to GTK+ 4 in the future. The changes should work in both GTK+ 3 and GTK+ 4.
I went through each change to confirm that the UI looks the same as before, but there is a risk of having made mistakes.
There were two cases where I didn't update the code:
- The tabs in
src/gnome-software.ui
seemed to align incorrectly when I changed the packing. - There is some code in
src/gs-common.c
that modifies the behaviour of GtkMessageDialog - I don't think this can be done in a way that supports both GTK+ 3 and GTK+ 4 (and the code is pretty questionable anyway...).