egg: bring multi-paned ported to GTK 4

This is out of libpanel, which already has it but with the dock stuff
removed (more like DzlMultiPaned). We probably still need some work to
handle natural allocation from children.

src/libsysprof-ui/meson.build

@@ -13,7 +13,11 @@ libsysprof_ui_public_sources = [
 ]
 
 libsysprof_ui_private_sources = [
+  'egg-handle.c',
+  'egg-paned.c',
+  'egg-resizer.c',
   'egg-three-grid.c',
+
   'pointcache.c',
   'rectangles.c',
   'sysprof-aid.c',