Reload swaybar/swaybg on config reload.

This works by tracking the pids of the child processes in the related
output container and terminating the processes and spawning new ones on
a config reload.

Should solve: #347

include/container.h

@@ -1,5 +1,6 @@
 #ifndef _SWAY_CONTAINER_H
 #define _SWAY_CONTAINER_H
+#include <sys/types.h>
 #include <wlc/wlc.h>
 typedef struct sway_container swayc_t;
 
@@ -81,6 +82,11 @@ struct sway_container {
 	char *class;
 	char *app_id;
 
+	// Used by output containers to keep track of swaybar/swaybg child
+	// processes.
+	list_t *bar_pids;
+	pid_t bg_pid;
+
 	int gaps;
 
 	list_t *children;