extend function expressions to observe function ctxt

This PR extends the function expression support in order to automatically re-calculate the function value when one of the property of the function context changes (i.e. any change on the 'this' object).
Of course the function expression also observes the function arguments and the function path.

NB: this PR only concerns function expressions used in element attributes. To support the function expressions in text node, we need to remove the *insert* node implementation (which are not used any longer as they have been replaced by components). As this may touch more files (compiler + runtime), I will do this in a separate PR when my 2 current PRs are integrated.