Adding support for gobject-introspection

Hey,

Lately I've been working on adding gobject-introspection support for
libabiword (patch attached), although it's been a mostly smooth
experience I found some miscellaneous issues:

* I noticed that abi_widget_get_selection() and abi_widget_get_content()
advertize that the text must be freed through g_free(), although it's
allocated through c++ memory allocation, this plays odd with the
introspection support as it expects strings to be malloc()'ed. I've
attached a patch to make it work as advertized.

* In xap_UnixTableWidget.cpp, the restart_widget() function crashes on
GTK+3, as the event handler really expects an meaningful event there. My
understanding is that this code tried to cope with returning the widget
to a regular state as it disappeared while being clicked on. since >=
2.8, GTK+ widgets handle GdkEventGrabBroken, which get triggered on
situations like this, so I think this code can be safely removed, or
cased to older versions.

* I can also see graphical oddities (like the AbiWidget disappearing
from the window briefly after being shown), but this is doesn't seem
to be related to python nor gobject-introspection, as I also reproduced
on a simple C client, I'm still investigating what could cause this.

Comments welcome,
  Carlos