Emit Show/Hide events when a layout is shown or hidden. (#63)

luigi/event.lua

@@ -47,6 +47,8 @@ Event.names = {
     'Press', -- A pointer button was pressed and released on the same widget.
     'Change', -- A widget's value changed.
     'WheelMove', -- The scroll wheel on the mouse moved.
+    'Show', -- A layout is shown.
+    'Hide', -- A layout is hidden.
 }
 
 local weakKeyMeta = { __mode = 'k' }

luigi/layout.lua

@@ -195,6 +195,7 @@ function Layout:show ()
     Backend.show(self)
     self.root:reshape()
 
+    Event.Show:emit(self, self)
     return self
 end
 
@@ -213,6 +214,7 @@ function Layout:hide ()
     self.isShown = nil
     Backend.hide(self)
 
+    Event.Hide:emit(self, self)
     return self
 end