gtk: Use the same behavior in Builder ctor overloads
gtK: Fix Builder xml definition loading

Gtk.Builder crashes, when AddFromString(string) is called with a string that
contains a BOM. Hence, if the buffer contins a BOM, remove it.
This fixes the crash with the Gtk# 3 project template in MonoDevelop
(robpvn/MonoDevelopGtkSharp3Template).