perf ui browser: Add ui_browser__show counterpart: __hide

So that the common tasks of providing a helpline at __run entry and
destroying the window and releasing resourses at exit can be abstracted
away, reducing a bit more the coupling with libnewt.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>