ActionHandler dialogs behave unexpectedly when Yes and OK buttons have the same label

[S. Zeid]

When the Yes and OK buttons have the same label, the ActionHandler will fail to perform the requested action after the user confirms the action.

Steps to reproduce:

1. Switch to a system language for which AppBackup's localizations for the "yes" and "ok" strings are the same (e.g. Korean).
2. Launch AppBackup.
3. Select an app or press the All button.
4. Choose an action.
5. Select "Yes" on the confirmation screen.

Expected behavior:

The action is performed.

Actual behavior:

The action is not performed, and the user is returned to the app list.

[S. Zeid]

[gui/ActionHandler] keep track of what stage of the action flow the user is in; fixes #1

→ <<cset e8808d13269f>>