perf ui browser: Introduce ui_browser__write_nstring()

perf ui browser: Introduce ui_browser__write_nstring()

To remove direct access to libslang functions, with the immediate goal
of implementing horizontal scrolling at the ui_browser level, but also
because we may at some point want to implement ui_browser with other UIs
in addition to the current libslang implementation.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-437ineavoejzou727mr9bxpi@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>