Reworked the PasswordGeneratorWidget.

It's loosely based on OS X PasswordAssistant. Generation happens as soon as a
change is made, and on open of the widget. A combobox has been added to allow
one to choose from some randomally-generated alternatives, and the UI is
generally been made a bit more compact.

Written by Michael Curtis <michael@moltenmercury.org> and revised by me.

Closes #119

https://github.com/keepassx/keepassx/pull/38
Move password generator widget to the right row.
Uncheck the correct widget in ChangeMasterKeyWidget.

We want to enable the password group but mask the password in the line edit.