Gtk.Builder crashes when AddFromString(string) is called with a string
that contains a BOM. Hence, if the buffer contains a BOM, just skip it.
This fixes the crash with the Gtk# 3 project template in MonoDevelop
(robpvn/MonoDevelopGtkSharp3Template).
In Windows builds of GTK+ 3.x, the dll filename is libgtk-3-0.dll.
We use this opportunity to use a common const in the DllImport
statement for all custom code.
There are no real code changes in this commit, just a lot of file
renaming and boilerplate additions.
A few .custom files are just removed, because the corresponding class in
GTK is gone, so they were not really used anymore.
Some files need to be re-indented, but that will be done in a separate
commit, so that git can track the renamed files correctly and not be
confused by all the changes.
