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.

meson.build

@@ -12,6 +12,8 @@ project(
 
 add_project_arguments(
 	[
+		'-DSWAY_SRC_DIR="@0@"'.format(meson.current_source_dir()),
+
 		'-DWL_HIDE_DEPRECATED',
 		'-DWLR_USE_UNSTABLE',