Gio::MemoryInputStream: Add add_data() taking a sigc::slot

* gio/src/memoryinputstream.[h|cc]: Add add_data(
const void*, gssize, const SlotDestroyData&). Deprecate
add_data(const std::string&) and add_data(const void*, gssize).
Bug #609946
Add Gio::MemoryInputStream test

* tests/giomm_memoryinputstream/main.cc: New file.
* tests/Makefile.am: Add the MemoryInputStream test.
Bug #609946
MemoryInputStream test: Small improvements

* tests/giomm_memoryinputstream/main.cc: Slightly better test of the
deletion of the data. More output in case of failure.
2.39.4
Merge branch 'master' of git://git.gnome.org/glibmm
JOHNE - Bump the version number to 2.39.04