Utils: Remove the text option for LayoutItem

This is converted to QLabels on the Qt side anyway, so we can can
just do that and avoid the special casing later for form layouts
later and gain the functionality for grids.

Use the opportunity to make the all mouse-selectable.

Change-Id: I37830b028b8d2987ea4925f364c79e905be725c8
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/layoutbuilder.h

@@ -62,7 +62,6 @@ public:
         QLayout *layout = nullptr;
         QWidget *widget = nullptr;
         BaseAspect *aspect = nullptr;
-        QString text;
         int span = 1;
         Qt::Alignment align;
     };