Fix autogen with latest gnome-common

Side-by-side use of IT_PROG_INTLTOOL and AM_GNU_GETTEXT is not supported.

https://bugzilla.gnome.org/show_bug.cgi?id=706835
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Fix a few warnings
Use the new gobject-introspection annotations

And fix warnings given by the newer version.
No longer use deprecated pygobject API
Remove autogenerated INSTALL file
Merge python2 and python3 extension tests

This will avoid further duplication when adding new tests.
Use g_test_trap_subprocess() instead of g_test_trap_fork()

g_test_trap_fork() is deprecated.
Assert if an engine is alive after g_test_run()

This can happen when adding new tests.
Print a debug message when a plugin loader is finalized

It is useful to know that they are actually being freed.
Assert an expectation of test_engine_get_default()
Add a since tag to peas_engine_prepend_search_path()
Assert that the plugin loaders are actually freed on shutdown

Useful to know that no extra refs are hanging around.
Various cleanups
Fix memory leak in python plugin loader
Fix incorrect free

When prgname is NULL we would try to free a constant string.
Only disable python warning redirections when not initialized

Python programs would want them to still be redirected.
Add a test for already an initialized python
Print a critical warning when attempting to mix Python versions

This prevents people from trying to load python3 plugins using
a python2 program and vice versa.