OSK for SIM PIN should only offer number entry

When the SIM needs to be unlocked I get presented with a dialog box with an entry for the PIN, along with a QWERTY OSK. The OSK should be a number-only keyboard.

I'm not sure which component displays the SIM PIN entry. Regardless, assuming it's a GtkEntry, calling gtk_entry_set_input_purpose with GTK_INPUT_PURPOSE_DIGITS should make the OSK number-only.

/cc @mohammed.sadiq @dorota.czaplejewicz @guido.gunther @sebastian.krzyszkowiak