Replace wlr_log with sway_log

This commit mostly duplicates the wlr_log functions, although
with a sway_* prefix. (This is very similar to PR #2009.)
However, the logging function no longer needs to be replaceable,
so sway_log_init's second argument is used to set the exit
callback for sway_abort.

wlr_log_init is still invoked in sway/main.c

This commit makes it easier to remove the wlroots dependency for
the helper programs swaymsg, swaybg, swaybar, and swaynag.

swaybar/render.c

@@ -5,7 +5,6 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>
-#include <wlr/util/log.h>
 #include "cairo.h"
 #include "pango.h"
 #include "pool-buffer.h"