Modified KPasswordDialog

[Andrew Coles]

I've made the following changes to KPasswordDialog after discussion on the 
list:

- added a minimum acceptable password length

- added a maximum acceptable password length, modifying the password
  strength meter code so it takes the maximum length into account when
  calculating the strength figure.

- added a password strength warning level: the user is given a warning if
  they enter a password whose strength is below this level

The patch can be found at:

http://cis.strath.ac.uk/~ac/kpassdlg.diff

As far as I can tell, I haven't broken binary compatibility, but someone in 
the know might want to check.

Comments / OK to commit?

Thanks,

Andrew