Cleanup and build with new core18 and gnome-3.28 platform

[fossfreedom]

I have had a go tidying up the snap.

It now builds and runs ... sort of ...

It saves the desktop results within the snap itself - i.e. doesnt display the contents of ~/Desktop

It doesnt open a terminal - suspect I am missing a stage-package complaining that it can't execute the child process x-terminal-emulator

But from a quick glance, you can create panels, files, link files, photos

So this is at least something someone can work with to cleanup and fixup

[spheras]

:open_mouth: WOW! thank you @fossfreedom!! Hope lot of users can benefit from the snap in the future!