Doc: Make Qt Quick demos accessible from Qt documentation

Add screenshots and brief descriptions. Integrate source code into
documentation.

Create new group, "qtquickdemos".

Change-Id: I92715b2cdd861b1ff2bdd28a640e63d2a220499e
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>