gcr-prompt: allow to set input purpose of password field

When e.g. entering smartcard or SIM PINs we want the input prupose to be set to GTK_INPUT_PURPOSE_DIGITS or GTK_INPUT_PURPOSE_PIN so e.g. a OSK can display a purely numeric layout. It would be great if the input purpose would be configurable via s.th. like:

gcr_prompt_set_password_input_purpose (GcrPrompt *prompt, GtkInputPurpose purpose);