Generates descriptors and events for signals automatically. Needs
changes to the Signal class in GObject and additionally a way of
detecting "run-after". Still a work in progress.

This commit allows for signals to be generated with the correct
syntax, and with the entire project compiling again. The codegen
is itself complete (minus some edge-case features that we may want
to consider down the road).

The final remaining step is to generate custom SignalArgs-based types
for signals with additional parameters.

…into GenerateSignals

This takes SignalArgs generation a step further, automatically
generating a field for each property. The final step is to be able
to marshal between GValue and the given field's concrete type.

We might want to consider avoiding GValue and Closures altogether if
we go down this route, since it adds an additional layer of overhead
with seemingly few benefits.

 - Explicitly activate nullable directive to avoid compiler error which is thrown if the nullable directive is not explicit activated in *.generated.cs files

…ocation)

 - Fix compile errors
 - Move SignalArgs indexer support to Object
 - Use Notebook as testclass for generic signals

Fix various parts of the quick-start demo, as well as add
significant amounts of commenting to serve as a self-contained
tutorial for learning Gir.Core.

QuickStart Demo

 - Support concrete types as sender (without null check in client code)
 - Support Events which just send empty EventArgs

 - Make SetArgs internal
 - Extract signal data only on request

Finally got this .editorconfig file. Configured to follow the default C# code spec, and compatible with StyleCop.

Apply .editorconfig code style