do not call basename_nosuffix() before using matching_unsuffixed_filename_exists_in() because the latter does that call itself.

As the code used to be, if we are testing for a candidate filename of aaa.bbb.ccc, the call strips .ccc and matching_... strips .bbb
resulting in a match with aaa, which is not intended at all.
add libardour infrastructure for "fade range" edit operation
add "fade range" operation, bound to alt-f at present by default