[web & desktop] Hide all characters in a TextField, when obscureText is true

[nilsreichardt]

Problem

In Flutter Web the last character of a TextField is visible, when obscureText = true. This should not be the case. Specially when your screen is shared with a beamer, etc. everybody can see your password.

Example:

How it should be

Every website hides all characters in a password field.

[yjbanov]

@HansMuller @goderbauer I removed the web flag because this is a general desktop problem. On desktop password fields generally do not show letters.