perf ui browser: Abstract some more slang operations

Browsers don't have to deal with absolute coordinates, just using (row,
column) and leaving the rest to ui_browser is better and removes one
more UI backend detail from the browsers.

Also shorten the percent_color setting idiom, removing some more direct
libslang calls.

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>