Makes QPlatformTheme::PasswordMaskDelay themable using QStyle

Creates new themable setting SH_LineEdit_PasswordMaskDelay which can be used
to override QPlatformTheme::PasswordMaskDelay. SH_LineEdit_PasswordMaskDelay defines number
in ms which the password character is visible when echoMode is set to Password

[ChangeLog][General][QStyle] Creates a new themable variable SH_LineEdit_PasswordMaskDelay

Change-Id: I26f0ddfda2b49bc293e00e8fcf62260cf4896ab2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>