bar: wip: define a 'backend' interface

Implement the current XCB backend in terms of this new interface.

bar/backend.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <stdbool.h>
+
+#include "../bar.h"
+
+struct backend {
+    bool (*setup)(struct bar *bar);
+    void (*cleanup)(struct bar *bar);
+    void (*loop)(struct bar *bar,
+                 void (*expose)(const struct bar *bar),
+                 void (*on_mouse)(struct bar *bar, enum mouse_event event,
+                                  int x, int y));
+    void (*commit_surface)(const struct bar *bar);
+    void (*refresh)(const struct bar *bar);
+    void (*set_cursor)(struct bar *bar, const char *cursor);
+};