Provide hook to alter view executable before render in view widget

Rebased patch into an MR.

Tests are mostly passing now on the MR. At least the same tests that aren't failing on HEAD are now passing. Hiding patches.

This is looking good to me. I tried it out and it works brilliantly. The context data provided is very complete and helpful. One gotcha was that the Views display is not retained, so switching to another display cannot be done in the normal way (by calling ViewsExecutable::setDisplay('my_display')).

This is not a problem in practice since the widget configuration is also passed by reference in the alter hook, so it can be done by setting $configuration['view_display'] = 'my_display'.

Some very clever people involved here, all comments resolved. So let's merge this in finally.