Code standards improvement

[rozniak]

General issue for me to go through the project to get all the source up to the same sort of level, since some parts of the project are a bit scuffed from when I knew very little about GTK.

Some stuff to do:

• Add the necessary project desc / licence headers to the source
• Bin off most of the GObject boilerplate - make proper use of G_DECLARE_DERIVABLE_TYPE etc., follow best practices for GObject
• Use incoming shell extension UI API (sysdm - fill out remaining system info #393) in existing 'UI behaviour' code (host implement WinTCIShextUIHost and behaviours derive from WinTCShextUIController)
• Document order of code sections - improve some of the names like // GTK OOP BOILERPLATE (actually refers to GObject)

Perhaps some other bits... those are the main things for now.