Use a BoolExpr typedef to make operator const void* clearer.

* gtk/gtkmm/stockid.[h|cc]:
* gtk/src/iconinfo.hg:
* gtk/src/papersize.hg:
* gtk/src/textiter.hg:
* gtk/src/treeiter.hg:
* gtk/src/treepath.hg:
* gtk/src/treerowreference.hg: Use a BoolExpr typedef and add documentation
because not all people will know the purpose of operator const void*.
  This was suggested by Daniel Elstner.