GObject overview

The main goal is to create a new documentation story for GObject. Right now there is a lot of stuff missing and outdated in the current docs.

The idea is to walk through it from a more practical aspects and explain how to solve real use cases around GObject (get/set properties, inheritance, implementing/creating an interface, installing properties/signals...)

There is a draft on the GObject wikipage. There is a generic bug on improving the GObject overview documentation.

Goals:

• Decide what's useful from the current docs and what needs to be adapted.
• Decide the final structure of the document
• Try to get help/contributors to write the docs