UI: Ensure OBS launches when theme is missing

This change makes it so OBS will still launch even if a theme is
missing. This change also deprecates the "CurrentTheme" config key, and
is replaced by "CurrentTheme2".

This is because a previous change to make sure OBS fell back to the
System theme in case of a missing theme had been accidentally removed.
Changing the key prevents a new version of OBS setting a theme that
doesn't exist in an older version, which would prevent that version from
launching.

UI/window-basic-settings.cpp

@@ -2760,7 +2760,7 @@ void OBSBasicSettings::SaveGeneralSettings()
 		themeData = DEFAULT_THEME;
 
 	if (WidgetChanged(ui->theme)) {
-		config_set_string(GetGlobalConfig(), "General", "CurrentTheme",
+		config_set_string(GetGlobalConfig(), "General", "CurrentTheme2",
 				  QT_TO_UTF8(themeData));
 
 		App()->SetTheme(themeData.toUtf8().constData());