qt,qt5: Ensure that input field is large enough for generated password

* qt/pinentrydialog.cpp (PinEntryDialog::PinEntryDialog): Change
minimum width of password input field.
* qt5/pinentrydialog.cpp (PinEntryDialog::PinEntryDialog): Ditto.

Use 35ex as minimum width. This should ensure that the generated
password (which has 30 characters) is fully visible.