[glibmm] (6 commits) ...Merge branch 'wip/dboles/Binding-master' into 'master'

[Daniel Boles <dboles src gnome org>]

Summary of changes:

  95b57f7... Binding: Better explain purpose of SlotTransform, (*)
  4ed3ff9... Binding: Move SlotTypeTransform up and document it (*)
  cecbf3c... Binding: no point to set target value if got false (*)
  a429221... Binding: Use std::optional instead of out-ref+bool (*)
  5f9dc06... tests/glibmm_binding: Add basic test inc transform (*)
  419ce6c... Merge branch 'wip/dboles/Binding-master' into 'master'

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