xcb: Add helper methods for GetPropertyReply
xcb: Add helper methods for GetPropertyReply

Add helper methods for GetPropertyReply so you don't have to access a void
pointer.

https://bugzilla.gnome.org/show_bug.cgi?id=730230

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