PBD::strip_whitespace_edges() returns the empty string if the passed string is

PBD::strip_whitespace_edges() returns the empty string if the passed string is
all whitespace.  This allows for some mild code cleanup.  It's also declared in
the PBD namespace now.

sfdb-v4 implemented.  Instead of fields that go across all files, there is now
just one tag field where you can enter comma delimited tags.  Searching for
tags to follow soon.  I recommend trashing your current ~/.ardour2/sfdb file.

git-svn-id: svn://localhost/ardour2/trunk@1182 d708f5d6-7413-0410-9779-e7cbd77b26cf