glib-2.0: mark many constants as const instead of static
glib-2.0: mark many constants as const instead of static

Fixes bug 646143.

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