app: show the layer mode dropdown list properly.
Showing it was only displaying the top modes, with a lot of top space, and you had to slowly scroll down the list. This is the same as #2642 (as Alexandre noted in a comment), so I just use the same "fix" though I don't fully understand it. It feels more of a side effect of gtk_combo_box_set_wrap_width() working around a bug of GtkComboBox. So if anyone has a better fix and understand the issue, feel free to patch (maybe GTK+ directly?). In the meantime, it works well enough. :-)
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