Adds a case for handling the EVENT_CUSTOM_ACTIONSTART of type kActionReturn when in password input mode

This will make the key mapped to the "return" action (by default being the ENTER key) work for submitting the password. If the ENTER key is not mapped to some (other) action it can also be used to submit the password. There is still the issue of ScummVM setting letter keys (w for walk, l for look, u for use, t for talk) as default keymaps for "verb" in-game actions, which will not allow the player to enter those letters unless the player remaps the relevant keymaps. These letters are not used in the valid password.

This is a quick fix for issue https://bugs.scummvm.org/ticket/15917

A complementary fix would be to use non-letter keys for the default keymaps to the verb actions to free up the "l", "w", "u" and "t", but then the default keymaps for those won't be as intuitive.

An alternative fix would be disabling and re-enabling the keymapper for the "game-shortcuts" group but that comes with more potential issues:

• When should it be re-enabled to avoid inconsistent state if loading/saving within/outside the password input mode?
• Should the keymaps be re-organized to have a group with only the ScummVM-exclusive keymaps for verb actions (ie. which do not exist in the original game), so that we could isolate disabling and enabling those only, since the original game does allow other hotkey actions to be performed in the computer "scene" (including password input mode) like F1 (for the "help" popup) and the keys for save, load, restart, quit, pause, sound?
• The player could potentially still map letter keys to keymapper actions (enabled in the password mode), which would still be conflicting with the intended free typing mode for the password input.