sidebar: cleanup code, remove unnecessary basename and strdups

 - the code does not modify the buffers with paths, so strdup() is
   unnecessary

 - it seems that strchr() is enough (rather than fragile basename)

 - don't waste time for vfolders where we explicitly use folder
   description rather than folder path.

 - clean up the code to make the lines shorter

Signed-off-by: Karel Zak <kzak@redhat.com>