perf ui: Introduce routine ui_browser__is_current_entry

Will be used in more places in the new tree widget.

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>