closes #827, #825, #822, #821
It is useful for some kind of components to be able to use a completely dynamic slot expression.
This new t-set-slot directive is meant to replace t-set when we need to define the content of a sub slot. All new code should use that directive. The old t-set directive is still supported for now, but this should be removed when we publish Owl 2.0.
closes #640
Géry Debongnie