openbox/stacking: optimize stacking list operations to O(1)

[jopamo]

This patch adds a stacking_node pointer to the ObWindow struct, caching the window's position in the global stacking_list.

Previously, operations like stacking_remove, stacking_raise, and occlusion checks required calling g_list_find or iterating through the list, resulting in O(N) complexity. For setups with many windows, this created unnecessary CPU overhead during focus changes and restacking.

By utilizing the cached GList node:

1. Window removal becomes O(1) via a new helper stacking_detach_node.
2. Occlusion checks start iteration immediately from the window's position.
3. Restacking operations avoid linear scans to find the window.