Fix GUI margins & borders to make them more comfortable

[ewancg]

Before:

After:

• Menu bar no longer sticks out because of (potentially) improperly assigned border
• Group boxes are no longer squeezed up against each other
• Spacing within settings widgets is no longer inconsistent

[ibancg]

Hi, thanks for the contribution, all the UI changes look fine to me.

I will be happy to merge those, but I get two new GTK warnings:

(lingot:476803): Gtk-CRITICAL **: 22:20:24.738: gtk_paned_set_position: assertion 'GTK_IS_PANED (paned)' failed
(lingot:476803): Gtk-CRITICAL **: 22:20:24.738: gtk_paned_set_position: assertion 'GTK_IS_PANED (paned)' failed

It seems this undoes previous commit 031e619, where I was introducing resizable panels with GtkPaned, now gone. Was this intentional?, it's like if you started your contribution from version 1.1.0 instead of 1.1.1.

[ewancg]

I didn't mean to overstep any recent commits. I was working on the latest release, solely because it had the automake files handy. After I was finished I just copied the glade files over into my fork, which was on the latest commit. I was using 1.1.1, though. Remedial commits coming shortly.

[ibancg]

True, 1.1.1 was the last one released, not 1.0.0. All fine now, thanks.