We want the notification screen stack to be displayed from oldest to newest, we want all windows to be animated when deleted and pressing ESC should remove the oldest notification first

Lots of copy/paste; main MythScreenStack should handle this, but as to not break anything, limit the changes to the notification center.

Fixes bug #11683
Fixes bug #11685
Do not re-create a notification center if one has already been created.

Reloading the theme would cause a 2nd notification center to be created the previous one leaking
Fixes bug #11684
Do not reload the entire notification screen whenever we're about to display it.

This caused the images to be reloaded 70 times per second, preventing images to be loaded in the background
Add flag to mark if screen needs to be refreshed or not.