Adjust naming scheme of generated GTypes

In preparation for the upcoming changes.
Move all custom enum/flags GTypes into gperl-gtypes.[hc]
Create a GType for GSpawnFlags and register it with the bindings

Needed for libvte.
Hush some compiler warnings
Tests: do not try to modify a read-only value

t/7.t was failing on perl 5.19.4 because it attempted to assign to undef.
Improve diagnostics for errors in signal accumulators
Revert the change to the ref counting of initial wrappers of custom subclasses

This reverts the commit "Ensure timely destruction of initial wrapper of custom
subclasses", eb6ff1b1e6e7fb5dcef4aa131cae9d95d2049460.

It turns out that the change causes issues with subclasses inheriting from
GInitiallyUnowned.  Instances of classes derived from GInitiallyUnowned would
be destroyed prematurely when code in INIT_INSTANCE manages to sink the
initial, floating reference.

Reported by Kevin Ryde.
Add support for G_SPAWN_DEFAULT, if available
Avoid deprecated API with modern GLib

We want to keep the ability to build perl-Glib with ancient versions of
GLib, but we also want to avoid compiler warnings when compiling against
modern versions of GLib. This means avoiding deprecated API, and trying
to keep up with the development of the library as much as possible.

https://bugzilla.gnome.org/show_bug.cgi?id=711840
Update copyright dates in recently changed *.xs files
Stable release 1.303
Fix compilation against glib < 2.34.0

https://rt.cpan.org/Ticket/Display.html?id=91313
Stable release 1.304