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.

sway/debug-tree.c

@@ -1,7 +1,6 @@
 #include <pango/pangocairo.h>
 #include <wlr/backend.h>
 #include <wlr/render/wlr_texture.h>
-#include <wlr/util/log.h>
 #include "config.h"
 #include "sway/debug.h"
 #include "sway/input/input-manager.h"