Make sure that fullscreen clients are on correct layer

[fbushstone]

Closes Issue 487.
Should actually fix the issue rather than just bandaid over it like by earlier PR.

Clients were that had fullscreen enabled were reparented to the floating layer rather than the fullscreen layer. This should allow clients that have fullscreen enabled to be set to the correct layer.

[fbushstone]

This change could also be implemented by changing layers[c->isfloating ? LyrFloat : LyrTile] to layers[c->isfullscreen ? LyrFS : c->isfloating ? LyrFloat : LyrTile] in line 2031. The earlier commit showcases another way to go about this, but I think this method should be a bit cleaner.

[sevz17]

Thanks!