[gtk] (6 commits) ...Merge branch 'wip/exalm/buttons' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  f3befe5... button: Reset style classes when setting arbitrary children (*)
  76df60d... menubutton: Reset label when setting icon name and vice ver (*)
  51a7545... menubutton: Remove a random g_return_if_fail() (*)
  ac3b246... menubutton: Add a way to always show arrow with image butto (*)
  47b5c68... menubutton: Don't hardcode label<->arrow spacing (*)
  bf97dfd... Merge branch 'wip/exalm/buttons' into 'master'

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