Add web color QLineEdit to QColorDialog.

This web color QLineEdit shows the hex number of the selected color.

Besides, it can be edited and accepts 3 and 6 digit hex numbers.

Change-Id: Idf629fbdc203fc099d446b31cbb49f2ff56be810
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Kevin Ottens <kevin.ottens@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>