[kcms/users] Offer to change kwallet password when changing login password

Right now, when a user changes their login password, their KWallet
password is not changed. For users whose KWallet password was the same
as their login password, this causes the two passwords to get out of sync,
and then the user will be prompted to unlock their wallet the next time
they log in. If they use their new password, it won't work. This is very
very frustrating if you don't know what's going on.

Now, when you change the login password of your own user account, and
you have a wallet named "kdewallet" (the default one) in your list of
wallets, you will be prompted to change the password of that wallet to
match the new login password. This is optional and you don't have to do
it, and you won't be prompted to do so if you don't have a wallet named
"kdewallet", which indicates that you have a customized KWallet setup
and you presumably know what you're doing.

BUG: 389030
FIXED-IN: 5.21