Fix QLineEditPrivate::shouldShowPlaceholderText()

As stated in commit 46106c4, placeholder text should not be shown
when an empty QLineEdit has visual preedit text fed by a composing
input method. The same condition applies to setPlaceholderText() in
the sense that there is no need to call update() when the placeholder
text is not even shown.

Change-Id: I2521a119b160e16ae0738d3575fea6d9939d6d2a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tasuku Suzuki <stasuku@gmail.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>