Proposed change for adding the ability to toggle password visibility in the password dialog.  This is similar to functionality in most password managers and is specifically added with the use case of password managers in mind - the password in that case is likely pasted twice into both the new password and confirm password fields.

If this is a welcome change, I am open to suggestions on rearranging the layout.

make check
bitcoin-qt -> Encrypt Wallet # verified the toggle shows the password as expected