core: replace window table and window stack with wm_tree

Because wm_tree tracks the entire window tree, it's able to replace
several old data structures we used to track windows: the window hash
table, the window stack, and the subwin table. So we got rid of those,
and fully switched to wm_tree.

Fixes window rules for window managers that don't put client window
directly under toplevel windows. This includes, according to people's
reports, at least i3 and KDE.

Fixed a couple small bugs:

* dbus was returning window ID as a boolean.
* window frame extents not cleared after its client window disappears.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

include/picom/backend.h

@@ -31,7 +31,7 @@ struct xvisual_info {
 };
 
 typedef struct session session_t;
-struct managed_win;
+struct win;
 
 struct ev_loop;
 struct backend_operations;