smart_wibox: Do not call set_position when the area reamin the same.

This is a band-aid on top of deeper issues when layout_changed is
wrongly called. This should prevent menus moving in random places
when the mouse is moved. Note that this commit wont stop them from
moving when their size really change.

This is harder to fix as all the "outside of the workarea" checks
still need to be performed and in many cases, such as async loaded
menus, the move itself is really necessary while in others it is
unwanted.

smart_wibox.lua

@@ -71,9 +71,13 @@ function main_widget:layout(context, width, height)
     if self.widget then
         local w, h = wibox.widget.base.fit_widget(self, context, self.widget, 9999, 9999)
         glib.idle_add(glib.PRIORITY_HIGH_IDLE, function()
+            local prev_geo = self._wb:geometry()
             self._wb.width  = math.ceil(w or 1)
             self._wb.height = math.ceil(h or 1)
+
+            if self._wb.width ~= prev_geo.width and self._wb.height ~= prev_geo.height then
                 set_position(self._wb)
+            end
         end)
         return { wibox.widget.base.place_widget_at(self.widget, 0, 0, width, height) }
     end