QComboBox had no option to tell the user that he must select an item -
there was no placeholder text like e.g. in QLineEdit. This feature is
widely used in html forms so we should support it also.
Therefore add a new property 'placeholderText' to specify a text which
should be shown when the current selected index is invalid.
[ChangeLog][QtWidgets][QComboBox] QComboBox got a new property
'placeholderText'
Change-Id: If6dac45c9f43455474e267907b0b0d893301c611
Fixes: QTBUG-1556
Fixes: QTBUG-2776
Fixes: QTBUG-77141
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
360df2cf74