[finished] More (display) modes for restarting

- Use a named enum instead of a collection of booleans
 - Support old-style configuration but complain about it
 - Update AppImage config as well

The new setup allows four different restart modes: never,
always, user-unchecked and user-checked. The user-modes
are interactive and give the user a choice (defaulting to
unchecked-don't-restart and checked-do-restart respectively).
The non-interactive versions vary in how they are
displayed.

data/config-appimage/modules/finished.conf

@@ -1,14 +1,24 @@
 # Configuration for the "finished" page, which is usually shown only at
 # the end of the installation (successful or not).
 ---
-# The finished page can hold a "restart system now" checkbox.
-# If this is false, no checkbox is shown and the system is not restarted
-# when Calamares exits.
-restartNowEnabled: true
-
-# Initial state of the checkbox "restart now". Only relevant when the
-# checkbox is shown by restartNowEnabled.
-restartNowChecked: false
+# Behavior of the "restart system now" button.
+#
+# There are four usable values:
+#   - never
+#       Does not show the button and does not restart.
+#       This matches the old behavior with restartNowEnabled=false.
+#   - user-unchecked
+#       Shows the button, defaults to unchecked, restarts if it is checked.
+#       This matches the old behavior with restartNowEnabled=true and restartNowChecked=false.
+#   - user-checked
+#       Shows the button, defaults to checked, restarts if it is checked.
+#       This matches the old behavior with restartNowEnabled=true and restartNowChecked=true.
+#   - always
+#       Shows the button, checked, but the user cannot change it.
+#       This is new behavior.
+#
+# The three combinations of legacy values are still supported.
+restartNowMode: user-unchecked
 
 # If the checkbox is shown, and the checkbox is checked, then when
 # Calamares exits from the finished-page it will run this command.