awful.tooltip:show() - swap timer.started and timer_function call

This is needed because if async code is run inside of a tooltip timer func the started property may not still be false.
The current version causes random spurious timer already started errors.

lib/awful/tooltip.lua

@@ -172,8 +172,8 @@ local function show(self)
     if self._private.visible then return end
     if self.timer then
         if not self.timer.started then
-            self:timer_function()
             self.timer:start()
+            self:timer_function()
         end
     end
     set_geometry(self)