Add optional support for voxel-engine and voxel-debug

Still can be used independently as before, but now if the 'game' parameter
(which is optional) is voxel-engine with game.buttons set by kb-bindings,
it will be used automatically; also, if loaded through voxel-plugins and
voxel-debug is present, it will be loaded after (also loaded after
voxel-plugins-ui to appear further down in the gui) and the same gui will
be used automatically.