UI improvements

ui/UIFocusStack.gd

@@ -0,0 +1,31 @@
+class_name UIFocusStack extends RefCounted
+
+signal activeLayerChanged(layer:ClosableMenu)
+
+const Z_STEP:int = 10
+
+var _stack:Array[ClosableMenu] = []
+
+func push(layer:ClosableMenu) -> void:
+  if not _stack.is_empty():
+    _stack.back()._onFocusLost()
+  _stack.push_back(layer)
+  layer.z_index = _stack.size() * Z_STEP
+  layer._onFocusGained()
+  activeLayerChanged.emit(layer)
+
+func pop() -> void:
+  if _stack.is_empty(): return
+  var removed:ClosableMenu = _stack.pop_back()
+  removed.z_index = 0
+  removed._onFocusLost()
+  var next:ClosableMenu = top()
+  if next != null:
+    next._onFocusGained()
+  activeLayerChanged.emit(next)
+
+func top() -> ClosableMenu:
+  return _stack.back() if not _stack.is_empty() else null
+
+func isTop(layer:ClosableMenu) -> bool:
+  return top() == layer