atk/atk.symbols: Make up for the Missed Symbols

There were some added public APIs for AtkValue, which was not updated to
atk/atk.symbols, so make up for them.

https://bugzilla.gnome.org/show_bug.cgi?id=726216
autotools: Ensure Needed Sources Are Distributed

atk/atkprivate.h and tests/teststateset.c were missed from the source
distribution, so make up for them to fix the build.

https://bugzilla.gnome.org/show_bug.cgi?id=726216
Move DllMain() to atkprivate.c

Since the HMODULE that is initialized by DllMain() is to be used to for
get_atk_locale_dir(), which was recently moved to atk/atkprivate.c, move
DllMain() and its related items to atk/atkprivate.c as well.  This will fix
the ATK build on Windows.

https://bugzilla.gnome.org/show_bug.cgi?id=726216