Pass return values through the GObject.Signal.emit wrapper

Return the result of GObject.emit from the Signal decorators wrapping.
Update unittest for decorated return type to use skipUnless for Python 3.
Add test for Signal decorator return type.

https://bugzilla.gnome.org/show_bug.cgi?id=705530
Add accumulator and accu_data arguments to GObject.Signal decorator

Update __init__, __call__, and copy methods to accept and pass
accumulators and associated user data through them. Update
accumulator unittests to use Signal decorators for testing accumulator
pass throughs. Verified the __gsignals__ dictionary accepts None
as valid values for accumulator and accu_data so specialization
for these arguments is not necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=705533
Use PyLong_Type.tp_new for GLib.Pid

For GLib.Pid the original implementation for __new__ needs to be used,
as it is able to initialize the integer correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=705451
Create GLib.Pid in the same way on python 2 and 3

https://bugzilla.gnome.org/show_bug.cgi?id=705451
NEWS: retroactively fix last version number
release 3.9.90
configure.ac: post-release bump to 3.9.91
Fix PEP-8 errors in propertyhelper.py

https://bugzilla.gnome.org/show_bug.cgi?id=706319

Signed-off-by: Martin Pitt <martinpitt@gnome.org>