Password prompt: add a show password button like the password stage

allow_show_password: When enabled, provides a 'show password' button with the password input field.

the setting is available only in the stage Password, so if you want to have username and password on the same form (not a stage password) it is not possible to enable it.

IMHO, allow_show_password should be a global setting: if True the button should be available on each password request; in this way the user interface could be more coherent and the code simpler
```[tasklist]
### Tasks
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