[gtk] (9 commits) ...headerbar: Disconnect signal handlers when children are removed

[Timm Bäder <baedert src gnome org>]

Summary of changes:

  f8303c7... testpopover: Plug two GtkBuilder leaks (*)
  e5f1ff6... popover: Use a bin layout for the contents gizmo (*)
  07d1ea4... cssimagerecolor: Avoid copying colors (*)
  3180cdb... cssdimensionvalue: Add more common pixel values (*)
  fb0b0dd... cssdimensionvalue: Add 50% singleton (*)
  cd3cd64... cssdimenstionvalue: Add common degree value singletons (*)
  edf56b4... css: short-circuit gtk_css_dimension_value_try_add (*)
  c2306d3... inspector: Select an object when activating it (*)
  d572b5d... headerbar: Disconnect signal handlers when children are rem (*)

(*) This commit already existed in another branch; no separate mail sent