feat(wm): add stackbar for multi-window containers

This commit introduces the stackbar feature through careful extracting
and refactoring of code from the Komorebi-UI hard-fork.

Unfortunately on the fork, this feature was not implemented using atomic
commits, which resulted in the implementation here being more of a
"reinterpretation" than a lift-and-shit of the referenced code.

Nevertheless, this commit represents a working version of the stackbar
feature.

resolve #681

komorebi/src/monitor.rs

@@ -71,7 +71,7 @@ impl Monitor {
             if i == focused_idx {
                 workspace.restore(mouse_follows_focus)?;
             } else {
-                workspace.hide();
+                workspace.hide(None);
             }
         }