Introduce a user scripts mechanism

Allowing programmatic injection of JavaScript to accomplish
all sorts of tasks on the render process side.
This API gives control over the point during the loading phase
at which the script is run, whether it is run on sub-frames or
not, as well as the JavaScript world it is run in (either the
page's main world, or an arbitrary isolated world).

This only has the Widgets API. The Quick API, tests and docs
are coming in separate patches

Change-Id: Ia1c79f68f8dfd4d964281d9723d09062ed7abe46
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>