Allow command chaining for :ClearSwapList and :SwapIdea

Especially for :ClearSwapList, one may want to put this into
    let b:undo_ftplugin .= '|ClearSwapList'

To enable appending other such commands, the command must support chaining, and because it doesn't take arguments, this is trivially done through -bar.
Retire :SwapListLoadFT.

This command doesn't make sense when :filetype plugin on (which most users have), as the filetype load mechanism automatically will source the swap list script when the filetype is set. It has never been documented, anyway. This may have been introduced to restore the current filetype's swap lists after opening of another buffer clobbered them, but that has now been addressed by using buffer-local variables.
Merge branches 'allow-command-chaining' and 'retire-SwapListLoadFT'