Reworked PasswordGeneratorWidget

The design of the widget has been based loosely on OS X' PasswordAssistant.  For clarity, I've attached some screenshots here:

Reworked dialogue, with passwords shown
![screen shot 2013-12-30 at 2 03 43 pm](https://f.cloud.github.com/assets/1663209/1820687/1649b298-70ff-11e3-9f26-9b11c0a85487.png)

A selection of 10 (by default) alternative passwords.  Note that the previously entered / generated password is kept at the top of the list in case the user changes their mind.
![screen shot 2013-12-30 at 2 03 59 pm](https://f.cloud.github.com/assets/1663209/1820688/164b78da-70ff-11e3-8581-bee1cb0c584d.png)

The alternative passwords chooser with passwords not shown.  It's less useful like this, but preserves the user's desire for privacy.
![screen shot 2013-12-30 at 2 04 09 pm](https://f.cloud.github.com/assets/1663209/1820689/164c7276-70ff-11e3-84ce-8d16f63b9541.png)

Generation also happens as soon as a change as made to the settings and on opening of the widget.  This makes it really quick to generate a password if the settings are already how you like them - just click generate and then Accept.

Because generation is now automatic, I removed the explicit button.

The "eye" icon is taken from here: https://www.iconfinder.com/icons/211739/eye_icon#size=128