Add a screen color picker button to QColorDialog

Add a new button with which any color present on the screen can be picked
up. This feature is very useful in any image processing application.

Notice:
--I have setMouseTracking to true because I consider that it makes much
easier to grab the desired color.
--I have set the cursor manually because for some unknown reason the
cursor wouldn't change with grabMouse(Qt::CrossCursor).

Task-number: QTBUG-14332

Change-Id: I39e3543d3ed55276f43d569a2f03087bbf89b27a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Kevin Ottens <kevin.ottens@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
List the URL schemes supported by QNetworkAccessManager

Introducing a new method which allows us to know before hand if an URL
scheme will be supported by QNetworkAccessManager. It is especially useful in
combination with QFileDialog URL based methods to pass this list as the
allowed schemes the user can select in the dialog.

Change-Id: If625b045e87959bfd78fea2c9213b69caf506886
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>