build: Fix opaquetest build on Windows

Create 2 makefile variable to facilitate build on Windows:
GENERATED_SOURCES_FILES to match files created by the code generator,
and GENERATED_SOURCES_OPTION for the corresponding option to be passed
to the compiler.

Also adapt the makefile for the valtest sample to use these variables.
sample: Fix GtkDemo/DemoImages.cs on Windows

Calling .PeekChar on a binary stream can cause an ArgumentException
because it tries to decode the byte.
Update the Windows installer infrastructure for GTK+ 3

Port the current installer infrastructure from the gtk-sharp-2-12
branch, adapting it as needed. Credits go to Duncan Mak for
maintaining the 2.12 installer in the past few years.

When building on Windows, if the WiX toolset is available, the msi is
generated automatically at the end of the build.