Open the About popup the first time a new version starts

The last version the About popup was shown for is saved as last_seen_version in
settings.cfg. When the game starts with another version, About opens to show
what is new.

data/settings.gd

@@ -26,6 +26,7 @@ const SAVED_SETTINGS: Dictionary[String, Array] = {
 		&"inspiration_list_speed",
 	],
 	"window": [&"fullscreen"],
+	"app": [&"last_seen_version"],
 }
 ## When this environment variable is set, the settings files are kept in that folder
 ## instead of the user's config folder. The tests set it so they never touch the real files.
@@ -83,6 +84,9 @@ var character_select_open: bool = false
 ## [code]MusicServerClient[/code]. Saved in the [code][queues][/code] section of
 ## [method saved_settings_path], so the game goes on with the same queue after a restart.
 var queue_ids: Dictionary[String, String] = {}
+## The version the game last showed the About popup for. When it is not [member version],
+## this version is new and the main window opens the About popup to show what is new.
+var last_seen_version: String = ""
 
 ## The game version, set in Project Settings > Application > Config > Version.
 ## Releases are made from it, see RELEASING.md.