Call g_signal_handlers_destroy on gtypeinstance finalizer
Call g_signal_handlers_destroy on gtypeinstance finalizer

Fixes bug 708130

flobrosch/vala-fixes,flobrosch/vala-fixes,GNOME/vala,spidermonkdust/vala,GNOME/vala,spidermonkdust/vala,frida/vala,rvu95/vala,rvu95/vala,spidermonkdust/vala,frida/vala,flobrosch/vala-fixes,frida/vala,frida/vala,GNOME/vala,frida/vala,rvu95/vala,GNOME/vala