Fix 'make check' with clang++ and --enable-warnings=fatal

* tests/test_cpp11_lambda.cc:
* tests/test_deduce_result_type.cc:
* tests/test_track_obj.cc: Don't define functions which are not used.
The clang++ compiler considers unused functions an error, when libsigc++ is
configured with --enable-warnings=fatal. Bug #724496.
Update MSVC_Net2010

* MSVC_Net2010/filelist.am: Add new project files.
* MSVC_Net2010/libsigc++2.sln: Add new projects.
* MSVC_Net2010/libsigc++2.vcxproj.filters:
* MSVC_Net2010/libsigc++2.vcxproj: Update the list of header files.
* MSVC_Net2010/tests/test_*/test_*.vcxproj.filters:
* MSVC_Net2010/tests/test_*/test_*.vcxproj: Add testutilities.[h|cc].
* MSVC_Net2010/tests/test_accum_iter/:
* MSVC_Net2010/tests/test_bind_ref/:
* MSVC_Net2010/tests/test_bind_refptr/:
* MSVC_Net2010/tests/test_copy_invalid_slot/:
* MSVC_Net2010/tests/test_cpp11_lambda/:
* MSVC_Net2010/tests/test_custom/:
* MSVC_Net2010/tests/test_lambda/:
* MSVC_Net2010/tests/test_limit_reference/:
* MSVC_Net2010/tests/test_slot_disconnect/:
* MSVC_Net2010/tests/test_track_obj/: New test case projects. Bug #724496.
Add a maintainer in the DOAP file
Update the signal_impl::notify() documentation

* sigc++/signal_base.h: Update the signal_impl::notify() documentation.
It should have been updated when notify() was updated. Bug #167714.
signal_impl::notify(): Don't delete signal_impl during erase()

* sigc++/signal_base.cc: signal_impl::notify() and sweep(): Don't delete
signal_impl while its slot list is being manipulated. Without this fix,
tests/test_bind_refptr crashes in MS Visual C++ in debug mode.
The behaviour without the fix is risky, and shall be avoided. Bug #564005.
Replace visit_each() overloads by struct visitor<>

Note: This patch breaks API for some users, but it does not break ABI.
Only users who have added their own visit_each() overloads are affected by
the API break. Their programs will still compile, but there will be run-time
errors, if they rely on auto-disconnection of slots.
Updated instructions for users who implement their own adaptors are found in
the description of sigc::adapts<>.

* sigc++/adaptors/bound_argument.h:
* sigc++/adaptors/macros/adaptor_trait.h.m4:
* sigc++/adaptors/macros/bind.h.m4:
* sigc++/adaptors/macros/bind_return.h.m4:
* sigc++/adaptors/macros/compose.h.m4:
* sigc++/adaptors/macros/exception_catch.h.m4:
* sigc++/adaptors/macros/hide.h.m4:
* sigc++/adaptors/macros/retype.h.m4:
* sigc++/adaptors/macros/retype_return.h.m4:
* sigc++/adaptors/macros/track_obj.h.m4:
* sigc++/adaptors/lambda/macros/base.h.m4:
* sigc++/adaptors/lambda/macros/group.h.m4:
* sigc++/adaptors/lambda/macros/operator.h.m4:
* sigc++/functors/macros/mem_fun.h.m4:
* sigc++/macros/limit_reference.h.m4:
* sigc++/visit_each.h: Replace overloads of visit_each<>() by
specializations of struct visitor<> with a specialized member function
do_visit_each<>(). Qualify all calls to visit_each() with sigc:: to avoid
argument-dependent lookup.
* sigc++/functors/macros/slot.h.m4: Qualify the calls to visit_each_type()
with sigc:: to avoid argument-dependent lookup.
* tests/test_functor_trait.cc: Fix the expected result now when the test
really works.
Thanks to Ryan Beasley <rbeasley@vmware.com>, bug #724496.
Tests: Add test_visit_each

* tests/.gitignore:
* tests/Makefile.am: Add test_visit_each.
* tests/test_visit_each.cc: New file. Bug #724496.
Add test_visit_each to MSVC_Net2010

* MSVC_Net2010/filelist.am: Add new project files.
* MSVC_Net2010/libsigc++2.sln: Add new project.
* MSVC_Net2010/tests/test_visit_each/: New test case. Bug #724496.
Documentation: Emphasize that signals are ref counted

* sigc++/signal_base.h: Emphasize that a sigc::signal holds a pointer to
a ref counted signal_impl. Bug #611941.