perf ui: Reimplement the popup windows using libslang

Just another step in stopping the use of libnewt in perf.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
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-vtxnmz1t1807ykprapnk9njl@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/debug.h

@@ -29,5 +29,6 @@ int ui_helpline__show_help(const char *format, va_list ap);
 
 void ui__warning(const char *format, ...) __attribute__((format(printf, 1, 2)));
 void ui__warning_paranoid(void);
+void ui__error(const char *format, ...) __attribute__((format(printf, 1, 2)));
 
 #endif	/* __PERF_DEBUG_H */