Makes PasswordMaskDelay configurable through TextInput

Adds a new property to TextInput which can be used to override QPlatformTheme::PasswordMaskDelay.
The new property is TextInput.passwordMaskDelay and takes in delay (ms) which time character
is shown before masking it. This is only when echoMode is set to TextInput.Password

[ChangeLog][QtQuick][TextInput] Added passwordMaskDelay property

Change-Id: I52812b883db11fdd21b25154887c51df8a44f69c
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>