perf ui: Introduce ui_browser->seek to support multiple list structures

So that we can use the ui_browser on things like an rb_tree, etc.

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>