move trigger code from Window to Panel

where the drag/resize code actually is. Panels can be dragged around the
parent screen without them being a Window

library/lua/gui/widgets.lua

@@ -441,6 +441,9 @@ end
 -- if self.autoarrange_subviews is true, lay out visible subviews vertically,
 -- adding gaps between widgets according to self.autoarrange_gap.
 function Panel:postUpdateLayout()
+    -- don't leave artifacts behind on the parent screen when we move
+    gui.Screen.request_full_screen_refresh = true
+
     if not self.autoarrange_subviews then return end
 
     local gap = self.autoarrange_gap
@@ -487,10 +490,6 @@ Window.ATTRS {
     draggable = true,
 }
 
-function Window:postUpdateLayout()
-    gui.Screen.request_full_screen_refresh = true
-end
-
 -------------------
 -- ResizingPanel --
 -------------------