Buffer local swaplist

With only one global `g:swap_lists`, setting the swaplist for one filetype overrides the previous filetype's list. This breaks swapping with multiple filetypes in parallel (e.g. a PHP and CSS file); the last opened filetype "wins".

Instead, a buffer-local variable should be used. Each buffer has its own swaplist (according to its filetype / manual `:SwapList`) definition then.