Improve support for var properties

Improve support for var properties

This commit changes the semantics of function assignment in QML.
Previously, function assignment was interpreted as binding assignment.
Now, function assignment is interpreted as function assignment, and
therefore fails for all property types other than "var" properties.

To support imperative binding assignment, a new function was added to
the Qt object: Qt.binding(function) which takes a single function
parameter and returns a function object which will be interpreted as
an assignable binding expression by the QML engine.

Finally, this commit also slightly changes the semantics of var
properties in that the "special" JavaScript values of null and
undefined may be assigned to var properties, rather than being
interpreted as reset requests.

Task-number: QTBUG-21842
Change-Id: Iee99a878b9badf0fb76e983da7ebfa493f55ceb5
Reviewed-by: Matthew Vogt <matthew.vogt@nokia.com>