Add option to shutdown to mod updater screen

[Wartori54]

Fixes #249 by adding two buttons to the screen, one to shutdown and one to restart.

Some translatable text needed to be added or modified, I am unsure on how to deal with missing translations so currently only the English.txt file was modified. Please notify me if this need to be changed.

Also the Update method was rewritten and spliced up to prevent too many layers of indentation.

[caeyo]

Very minor nitpick but just on the language, I'd go with the two actions being:

• Restart
• Exit

Exit just matches the phrasing the game uses in the main menu, mentioning Celeste is probably pretty redundant, and the ordering is super duper nitpicky but I guess it feels off to me because restart has been the default for a while and exit is also more "final". Simplifying down to Exit also means that the vanilla dialog translations can be used for that entry, if that helps

[Wartori54]

Ok thats a good idea to minimize the translations needed.

[DemoJameson]

found two issue:

1. if only mod can be updated, after updating done, the restart button doesn't has focus
   

2. if more than one mod can be updated, after updating them one by one, the update all button doesn't replace to a text All mods have been updated.
   

[Wartori54]

Thank you @DemoJameson for your testing, those should be all fixed on latest commit. Really looking forward to getting this merged.