Kill QWinTaskbarProgress::IndeterminateState

Automatically turn it to an indeterminate progress bar if
minimum and maximum both are set to 0 - just like QProgressBar.

Change-Id: Ie20121a8feaf95dbd35c4ea9e6bf342ee147011a
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Replace QWinTaskbarProgress::NoProgressState with a visible-property

Calling show()/hide() or setVisible(false) is a bit more intuitive
and Qt-stylish than setState(NoProgressState).

Change-Id: I12138454299f53a09e37bb82b473163e49ac97b3
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>