[glib] (8 commits) ...Merge branch 'wip/otte/interface-types' into 'master'

[Philip Withnall <pwithnall src gnome org>]

Summary of changes:

  1f6c737... Suggest braces around empty body in ‘if’ statements (*)
  e1b5afb... gtype: Fix typo in API comment (*)
  98f0a5a... gtype: Add g_type_interface_instantiatable_prerequisite() (*)
  3f2a8d5... closure: Support generic marshals for interface returns (*)
  458e3b7... value: Allow automatic transforms to/from interfaces (*)
  52357aa... Add a binding test involving interfaces (*)
  0b1f909... Add a test for signals returning interface types (*)
  5719d2c... Merge branch 'wip/otte/interface-types' into 'master'

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