perf ui: Separate showing the entries from running the browser

Another patch eroding the changes I had to move to a tree widget that
doesn't requires adding all entries in an existing list/tree structure
to a generic tree widget, but instead allows traversing just the entries
that should appear on the screen on a given moment.

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>