Assets copying, unify WORKING_DIRECTORY, fix linux build on some platforms

[tuxuser]

• Copy assets/ directory into after build
• Use same WORKING_DIRECTORY for all platforms (same directory as binary)

This yields the following structure

bin/Debug-linux/
bin/Debug-linux/OpenNeighborhood
bin/Debug-linux/OpenNeighborhood/OpenNeighborhood
bin/Debug-linux/OpenNeighborhood/assets
bin/Debug-linux/OpenNeighborhood/assets/icons/*
bin/Debug-linux/OpenNeighborhood/assets/fonts/*

• Fix linux build by explicitly linking with libGL and libGLEW (fixed gentoo build for me, should not have any negative side-effects)

[ClementDreptin]

Hey tuxuser !

Thank you so much for contributing! It seems like linking explicity with GLEW and OpenGL for linux breaks the remote build for the GitHub action (probably because they don't have the libraries installed on their machine). It should not be necessary anyway because the source code of glad is in OpenNeighborhood/vendor/Glad and it should take care of loading the OpenGL functions.

Can you please change OpenNeighborhood/premake5.lua accordingly? I'll put on comment on the related lines.

Thank you in advance!

Clément