[Development] Make WebKit an add-on

[Knoll Lars]

Hi,

Currently Qt WebKit and Qt WebKit Widgets are marked in our documentation
as Qt Essentials. I’d like to propose to move them to the add-on category
for 5.4.

There are several reasons for that, the main ones being:

* Qt WebKit is not supported on Android, iOS and WinRT
* We can’t develop it much further (esp we can't update with newer
versions of webkit), as the webkit project has moved into a different
direction
* Qt WebEngine is going to replace it over time

Unless someone has a very good reason against changing this, I’d like to
change the status of the 2 modules in time before the 5.4 branch is being
created.

Cheers,
Lars