ui browser: Add method to write graphical characters

To save typing on the switch char set slang stuff.

It also helps in removing more slang direct calls, wrapping them at the
ui_browser level, where at some point I'll try to implement those in
terms of GTK+.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-63yhb2htv9g3g1olmojzptkd@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>