This also demonstrates the use of Gtk.Builder to load .ui files.
Also simplify handling of resources in GtkDemo build: we don't need to
make a distinction between images and other resources.
The main difference with the C demo is that we can't use GResource, as
it's only in gio 2.32 and later, so '@import url("resource://...' won't
work. Instead, we use a separate CssProvider to apply reset.css.
When dealing with several namespaces there might be classes with the same name
(especially Global which is autogenerated in g-i based bindings). On generation
the file would be overriden by the last occurence in the xml. To encounter
this every namespace has it's own directory now. This also improves structure
a lot when dealing with big libraries.
Also do the necessary adaption for the build and the csproj files.
The sample writes the same PNG file over and over, with some pauses in
between iterations. Some indicative numbers are written to the console,
to help track performance and memory usage.
Feel free to add more elaborate drawing to this sample, to exercise more
of the cairo API.
Add a sample to illustrate the point of having a
GLibSynchronizationContext, but with the async/await code ifdef'ed out
because we don't require Mono 3.0 for now. Instead, provide a crude
equivalent of what would be generated by the compiler, to show that it
works.
