header-bar: Wrap start/end boxes into extra widgets

Merged Alice Mikhaylenko requested to merge wip/exalm/headerbar into main

Turns out, GtkSizeGroup forces the widget's final size and not allocated size. This mean that the we can't use halign/valign values to force smaller size and must wrap the start/end boxes into extra widgets for the size groups.

Fixes https://gitlab.gnome.org/exalm/libadwaita/-/issues/23

Edited by Alice Mikhaylenko

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