awful.tooltip: Use metatable magic to create wiboxes lazily

This might make the code a bit easier to follow. Unless you want to know where
the wibox is created...

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/tooltip.lua

@@ -58,30 +58,32 @@ local ipairs = ipairs
 -- @tfield boolean visible True if tooltip is visible.
 local tooltip = { mt = {}  }
 
-local function get_wibox(self)
-    if not self.wibox then
-        self.wibox = wibox(self.wibox_properties)
-        self.wibox:set_widget(self.marginbox)
+local instance_mt = {}
+
+function instance_mt:__index(key)
+    if key == "wibox" then
+        local wb = wibox(self.wibox_properties)
+        wb:set_widget(self.marginbox)
 
         -- Close the tooltip when clicking it.  This gets done on release, to not
         -- emit the release event on an underlying object, e.g. the titlebar icon.
-        self.wibox:buttons(abutton({}, 1, nil, self.hide))
+        wb:buttons(abutton({}, 1, nil, self.hide))
+        rawset(self, "wibox", wb)
+        return wb
     end
-    return self.wibox
 end
 
 -- Place the tooltip under the mouse.
 --
 -- @tparam tooltip self A tooltip object.
 local function set_geometry(self)
-    local wb = get_wibox(self)
     -- calculate width / height
     local n_w, n_h = self.textbox:get_preferred_size(mouse.screen)
     n_w = n_w + self.marginbox.left + self.marginbox.right
     n_h = n_h + self.marginbox.top + self.marginbox.bottom
-    wb:geometry({ width = n_w, height = n_h })
-    a_placement.next_to_mouse(wb)
-    a_placement.no_offscreen(wb, mouse.screen)
+    self.wibox:geometry({ width = n_w, height = n_h })
+    a_placement.next_to_mouse(self.wibox)
+    a_placement.no_offscreen(self.wibox, mouse.screen)
 end
 
 -- Show a tooltip.
@@ -97,7 +99,7 @@ local function show(self)
         end
     end
     set_geometry(self)
-    get_wibox(self).visible = true
+    self.wibox.visible = true
     self.visible = true
 end
 
@@ -112,7 +114,7 @@ local function hide(self)
             self.timer:stop()
         end
     end
-    get_wibox(self).visible = false
+    self.wibox.visible = false
     self.visible = false
 end
 
@@ -189,9 +191,9 @@ end
 -- @see set_text
 -- @see set_markup
 tooltip.new = function(args)
-    local self = {
+    local self = setmetatable({
         visible = false,
-    }
+    }, instance_mt)
 
     -- private data
     if args.delay_show then