Pave the way for our UI delegation strategy.

Starting with the context Menus for QtQuick.
Add default UI delegates as a subproject. We allow ourselves to use
Qt Quick Controls there for in order to get a nice "out of the box"
experience for things like context menus, dialogs, etc while leaving
the door open for system embedders to override this.

Opting out of the deployment of these QML files is still very primitive
but can be done by passing WEBENGINE_CONFIG+=no_ui_delegates at qmake
time.

Customization of context menus could be done via a qml component, which
is probably best kept in experimental for now while we address its
shortcomings.

Change-Id: I0705b20d5ddd3bb010f9371b65a181c6b02a03e1
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>