Big fat update (FNA3D, UWP, new functionality)

A new version of XAGE has been released on itch.io to alpha users (keys are still available to anyone who wants one):

• Automatic UWP project generation (i.e. you can now have your games target Xbox One & Windows Store).
• A new backend renderer: FNA3D.  The version currently used is still limited to OpenGL renderer but new ones - e.g. Directx11 and Vulkan - are on the way, which is an exciting development as it will further help future-proof the platform, as well as bring performance improvements.  To help with this (and free up some disk space on my poor laptop), I've set up some automated builds of the UWP dlls, via Github Actions.
• Recent engine changes around gamepad support, viewports & cameras etc, as well as various bugfixes.

All template games have been updated accordingly.  At some point documentation will need a lot of focus, but in the meantime let me know of anything that needs attention.  I've added a project on github to track all items considered necessary for a public release.